PERF: more flexible iso8601 parsing
closes #9714
closes #11899
closes #11871

makes ISO parser in C handle the following. I think 2 & 3 aren't actually iso8601 anymore, but close and unambiguous.
1. dates without '-' separator 
2. dates without space before tz
3. dates without leading 0s in month/day
   (this ONLY works if the date has separators, eg. `"2015-1-1"` parses, but `"201511"` doesn't because it's ambiguous)

asv results - adds a small amount of overhead to the standard case (`'2014-01-01'`)

```
    before     after       ratio
  [1ae6384 ] [e922b05 ]
     5.17ms     5.34ms      1.03  timeseries.timeseries_to_datetime_iso8601.time_timeseries_to_datetime_iso8601
     5.15ms     5.50ms      1.07  timeseries.timeseries_to_datetime_iso8601.time_timeseries_to_datetime_iso8601_format
-  111.89ms     5.27ms      0.05  timeseries.timeseries_to_datetime_iso8601.time_timeseries_to_datetime_iso8601_format_no_sep
-     2.02s     5.33ms      0.00  timeseries.timeseries_to_datetime_iso8601.time_timeseries_to_datetime_iso8601_nosep
-     2.97s   218.25ms      0.07  timeseries.timeseries_to_datetime_iso8601.time_timeseries_to_datetime_iso8601_tz_spaceformat

```

A question: is is worth the added complexity to the ISO 8601 C-parser to be able to handle the dates without separator (which is actually not really ISO 8601 anymore), if when passing the `format` is already as fast to parse this? 
Of course, you then need to specify this, but this could also be alleviated by making `infer_datetime_format=True` the default.
 Hard to say if it's worth it (re dates with separators), not sure it's a super common format.  But there is a substantial speedup even over providing a format to the strptime parser (20x in that asv bench).  

Not that it really matters, but from I've read dates without separators would still be considered ISO 8601, e.g. http://www.cl.cam.ac.uk/~mgk25/iso-time.html.
 how hard would it to allow a selection of separators

eg `-  .  /  \`
 (assume that if one is found it would have to repeat for the date portions, iow don't allow mixed seps)

w/o sacrificing ambiguity?
 also, let's add another issue to make `infer_datetime_format=True` as the default for `to_datetime` and `read_csv`. I don't think this will impact anything (except speed things up) and just make more things work.
 @chris-b1 Ah, yes, I missed that you also passed this to the ISO parser if that format is given. I misinterpreted the asv output so thinking that the generic format parser was as fast as the ISO parser, but indeed quite a bit slower.
 @jreback - I don't think it would be too bad to parse multiple separators, I'll try it out
 @chris-b1 just some minor comments on the benchmarks themselves here.

if the addtl separators is a bit complicated can do in another PR.

pls rebase on master as lots of PEP changes recently and do a `git diff master | flake8 --diff` check

ping if you want to merge now (else, pls open a new issue for the other sep chars if not)
 @jreback - updated for that asv note.  I was most of the way there already, so also updated to handle 
1. date separator can be any of `{'-', '.', '/', '\\', ' '}`, as long as consistent
2. leading 0s can also be omitted from the time components (must have `:` sep)

Passes the current test suite (adjusted for a couple timezone changes), not sure if there are other edge cases to try?

a couple perf regressions to figure out too

```
    before     after       ratio
  [567bc5ce] [82c5d7b9]
+  154.12ms      4.24s     27.50  plotting.plot_timeseries_period.time_plot_timeseries_period
+    1.82ms      2.19s   1203.52  timeseries.datetimeindex_infer_dst.time_datetimeindex_infer_dst
-     2.10s     5.14ms      0.00  timeseries.timeseries_to_datetime_iso8601.time_timeseries_to_datetime_iso8601_nosep
-     3.13s   223.79ms      0.07  timeseries.timeseries_to_datetime_iso8601.time_timeseries_to_datetime_iso8601_tz_spaceformat
```
 @jreback  - if you want to have a look, I've got everything working now.
 can you add tests for differnt seps (and invalid ones)?
 did you figure out the perf regressions?
 Well, I seemed to fix it, but didn't figure it out.  

`tz_localize` was very running very slow, but only a fresh install of pandas - wasn't slow if run a second time.  Also seemed to be fine if stepping through with `pdb`.  Undoing this [change](https://github.com/chris-b1/pandas/commit/5d565297cc653c80d0c3eaece864d457135c6876#diff-05de2950b2c86c9828531957e02d1b87L2021) fixed it - very unclear to me how that caused it though.
 hmm, I think this interferes with the `infer_datetime_format` ? haven't really stepped thru the code that recently.
 @jreback - updated.  Perf issue turned out to be unrelated - just some kind of caching that `pytz` does (also shows up on a fresh install of 0.17.1).

```
    before     after       ratio
  [5b5b2fe8] [5be82023]
   925.61us   929.00us      1.00  frame_methods.frame_assign_timeseries_index.time_frame_assign_timeseries_index
   145.86ms   145.63ms      1.00  plotting.plot_timeseries_period.time_plot_timeseries_period
     3.18ms     3.11ms      0.98  timeseries.dataframe_resample_max_numpy.time_dataframe_resample_max_numpy
     3.21ms     3.10ms      0.97  timeseries.dataframe_resample_max_string.time_dataframe_resample_max_string
     2.79ms     2.73ms      0.98  timeseries.dataframe_resample_mean_numpy.time_dataframe_resample_mean_numpy
     2.76ms     2.86ms      1.03  timeseries.dataframe_resample_mean_string.time_dataframe_resample_mean_string
     3.40ms     3.25ms      0.96  timeseries.dataframe_resample_min_numpy.time_dataframe_resample_min_numpy
     3.40ms     3.25ms      0.95  timeseries.dataframe_resample_min_string.time_dataframe_resample_min_string
   667.96us   661.20us      0.99  timeseries.datetimeindex_add_offset.time_datetimeindex_add_offset
     1.19ms     1.16ms      0.98  timeseries.datetimeindex_converter.time_datetimeindex_converter
     1.89ms     1.86ms      0.98  timeseries.datetimeindex_infer_dst.time_datetimeindex_infer_dst
     2.21ms     2.40ms      1.08  timeseries.datetimeindex_normalize.time_datetimeindex_normalize
   131.10us   127.90us      0.98  timeseries.datetimeindex_unique.time_datetimeindex_unique
   401.88us   395.18us      0.98  timeseries.dti_reset_index.time_dti_reset_index
   445.96us   462.25us      1.04  timeseries.dti_reset_index_tz.time_dti_reset_index_tz
    63.58ms    67.03ms      1.05  timeseries.period_setitem.time_period_setitem
     1.04ms     1.07ms      1.03  timeseries.timeseries_1min_5min_mean.time_timeseries_1min_5min_mean
     1.24ms     1.23ms      0.99  timeseries.timeseries_1min_5min_ohlc.time_timeseries_1min_5min_ohlc
    13.13ms    13.95ms      1.06  timeseries.timeseries_add_irregular.time_timeseries_add_irregular
     3.07ms     3.04ms      0.99  timeseries.timeseries_asof.time_timeseries_asof
     2.92ms     2.93ms      1.00  timeseries.timeseries_asof_nan.time_timeseries_asof_nan
    42.46us    43.79us      1.03  timeseries.timeseries_asof_single.time_timeseries_asof_single
    20.10us    23.67us      1.18  timeseries.timeseries_custom_bday_apply.time_timeseries_custom_bday_apply
    32.52us    33.25us      1.02  timeseries.timeseries_custom_bday_apply_dt64.time_timeseries_custom_bday_apply_dt64
    36.82us    41.39us      1.12  timeseries.timeseries_custom_bday_cal_decr.time_timeseries_custom_bday_cal_decr
    29.51us    30.93us      1.05  timeseries.timeseries_custom_bday_cal_incr.time_timeseries_custom_bday_cal_incr
    30.64us    31.04us      1.01  timeseries.timeseries_custom_bday_cal_incr_n.time_timeseries_custom_bday_cal_incr_n
    36.54us    35.03us      0.96  timeseries.timeseries_custom_bday_cal_incr_neg_n.time_timeseries_custom_bday_cal_incr_neg_n
    35.01us    34.81us      0.99  timeseries.timeseries_custom_bday_decr.time_timeseries_custom_bday_decr
    22.65us    24.13us      1.07  timeseries.timeseries_custom_bday_incr.time_timeseries_custom_bday_incr
   275.66us   262.08us      0.95  timeseries.timeseries_custom_bmonthbegin_decr_n.time_timeseries_custom_bmonthbegin_decr_n
   245.19us   255.52us      1.04  timeseries.timeseries_custom_bmonthbegin_incr_n.time_timeseries_custom_bmonthbegin_incr_n
   279.26us   292.99us      1.05  timeseries.timeseries_custom_bmonthend_decr_n.time_timeseries_custom_bmonthend_decr_n
   196.76us   202.95us      1.03  timeseries.timeseries_custom_bmonthend_incr.time_timeseries_custom_bmonthend_incr
   244.31us   237.53us      0.97  timeseries.timeseries_custom_bmonthend_incr_n.time_timeseries_custom_bmonthend_incr_n
     5.78ms     5.70ms      0.99  timeseries.timeseries_datetimeindex_offset_delta.time_timeseries_datetimeindex_offset_delta
    18.93ms    19.09ms      1.01  timeseries.timeseries_datetimeindex_offset_fast.time_timeseries_datetimeindex_offset_fast
    52.19ms    50.24ms      0.96  timeseries.timeseries_datetimeindex_offset_slow.time_timeseries_datetimeindex_offset_slow
    53.03us    59.37us      1.12  timeseries.timeseries_day_apply.time_timeseries_day_apply
    57.71us    58.96us      1.02  timeseries.timeseries_day_incr.time_timeseries_day_incr
    12.00ms    12.26ms      1.02  timeseries.timeseries_infer_freq.time_timeseries_infer_freq
     4.06ms     4.23ms      1.04  timeseries.timeseries_is_month_start.time_timeseries_is_month_start
   681.27ms   691.78ms      1.02  timeseries.timeseries_iter_datetimeindex.time_timeseries_iter_datetimeindex
    12.82ms    12.77ms      1.00  timeseries.timeseries_iter_datetimeindex_preexit.time_timeseries_iter_datetimeindex_preexit
      3.71s      3.65s      0.98  timeseries.timeseries_iter_periodindex.time_timeseries_iter_periodindex
    39.32ms    37.26ms      0.95  timeseries.timeseries_iter_periodindex_preexit.time_timeseries_iter_periodindex_preexit
    65.22us    65.56us      1.01  timeseries.timeseries_large_lookup_value.time_timeseries_large_lookup_value
    17.02ms    16.28ms      0.96  timeseries.timeseries_period_downsample_mean.time_timeseries_period_downsample_mean
     2.52ms     2.99ms      1.18  timeseries.timeseries_resample_datetime64.time_timeseries_resample_datetime64
     7.88ms     7.79ms      0.99  timeseries.timeseries_series_offset_delta.time_timeseries_series_offset_delta
    21.77ms    21.16ms      0.97  timeseries.timeseries_series_offset_fast.time_timeseries_series_offset_fast
    54.98ms    53.63ms      0.98  timeseries.timeseries_series_offset_slow.time_timeseries_series_offset_slow
    48.59us    47.74us      0.98  timeseries.timeseries_slice_minutely.time_timeseries_slice_minutely
     9.76ms     9.74ms      1.00  timeseries.timeseries_sort_index.time_timeseries_sort_index
     8.43ms     8.26ms      0.98  timeseries.timeseries_timestamp_downsample_mean.time_timeseries_timestamp_downsample_mean
    17.30us    16.55us      0.96  timeseries.timeseries_timestamp_tzinfo_cons.time_timeseries_timestamp_tzinfo_cons
    11.15ms    11.18ms      1.00  timeseries.timeseries_to_datetime_YYYYMMDD.time_timeseries_to_datetime_YYYYMMDD
     4.54ms     4.81ms      1.06  timeseries.timeseries_to_datetime_iso8601.time_timeseries_to_datetime_iso8601
     6.95ms     4.70ms      0.68  timeseries.timeseries_to_datetime_iso8601.time_timeseries_to_datetime_iso8601_format
-  114.53ms     4.67ms      0.04  timeseries.timeseries_to_datetime_iso8601.time_timeseries_to_datetime_iso8601_format_no_sep
-     1.98s     4.65ms      0.00  timeseries.timeseries_to_datetime_iso8601.time_timeseries_to_datetime_iso8601_nosep
-     2.91s   210.18ms      0.07  timeseries.timeseries_to_datetime_iso8601.time_timeseries_to_datetime_iso8601_tz_spaceformat
```
 some minor comments. ping when pushed / green.
 @jreback  - updated for your comments
 @chris-b1 thanks!

your PR's are always great!
