BUG: Timestamp cannot parse nanosecond from string
Fixes 2 problems related to `Timestamp` string parsing:
- `Timestamp` parsing results incorrect if input contains offset string (Closes #7833).
  
   NOTE: Also modified `Timestamp.__repr__` to display fixed timezone info, because this can be either `pytz` or `dateutil`.

```
# Result after the fix

# If string contains offset, it will be parsed using fixed timezone offset. Following results in 2013-11-01 05:00:00 in UTC (There is some existing tests checking this behaviour).
repr(pd.Timestamp('2013-11-01 00:00:00-0500'))
# Timestamp('2013-11-01 00:00:00-0500', tz='pytz.FixedOffset(-300)')

# If tz is specified simultaneously, it should convert the timezone. 
repr(pd.Timestamp('2013-11-01 00:00:00-0500', tz='America/Chicago'))
# Timestamp('2013-11-01 00:00:00-0500', tz='America/Chicago')

repr(pd.Timestamp('2013-11-01 00:00:00-0500', tz='Asia/Tokyo'))
# Timestamp('2013-11-01 14:00:00+0900', tz='Asia/Tokyo')
```
- `Timestamp` loses `nanosecond` info when parsing from `str`. (Closes #7878)

```
# Result after the fix

repr(pd.Timestamp('2001-01-01 00:00:00.000000005'))
# Timestamp('2001-01-01 00:00:00.000000005')

repr(pd.Timestamp('2001-01-01 00:00:00.000000005', tz='US/Eastern'))
# Timestamp('2001-01-01 00:00:00.000000005-0500', tz='US/Eastern')

repr(pd.Timestamp('2001-01-01 00:00:00.000001234+09:00'))
# Timestamp('2001-01-01 00:00:00.000001234+0900', tz='pytz.FixedOffset(540)')

repr(pd.Timestamp('2001-01-01 00:00:00.000001234+09:00', tz='Asia/Tokyo'))
# Timestamp('2001-01-01 00:00:00.000001234+0900', tz='Asia/Tokyo')

# Because non ISO 8601 format is parsed by dateutil, nanosecond will lost (no change)
repr(pd.Timestamp('01-01-01 00:00:00.000000001'))
# Timestamp('2001-01-01 00:00:00')

repr(pd.Timestamp('01-01-01 00:00:00.000000001+9:00'))
# Timestamp('2001-01-01 00:00:00+0900', tz='tzoffset(None, 32400)')
```

CC @cyber42 @ischwabacher @rockg @adamgreenhall

@sinhrks can you run a perf test and post if anything amiss (I don't think so, but just to be sure)
  As the original one gets little slower, modified the logic a little. Following is a perf before/after the modification. Looks varied, but no test gets constantly worse.

#### Perf with original logic in this PR (for ref)

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
read_parse_dates_iso8601                     |   3.5694 |   2.3197 |   1.5387 |
```

#### 1st run with current logic

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
read_parse_dates_iso8601                     |   2.3770 |   2.3616 |   1.0065 |
...
packers_read_pickle                          | 306.5277 | 254.1726 |   1.2060 |
frame_apply_lambda_mean                      |   9.4993 |   7.8301 |   1.2132 |
write_store_table_dc                         | 246.1410 | 197.7520 |   1.2447 |
frame_dtypes                                 |   0.2213 |   0.1760 |   1.2579 |
reindex_daterange_pad                        |   2.0043 |   1.5914 |   1.2595 |
read_store_table_mixed                       |  21.8923 |  17.3550 |   1.2614 |
frame_mask_bools                             |  33.6537 |  26.1207 |   1.2884 |
frame_get_numeric_data                       |   0.2956 |   0.2290 |   1.2908 |
indexing_dataframe_boolean_rows_object       |   1.3426 |   0.9793 |   1.3710 |
index_float64_div                            |   8.8797 |   6.3930 |   1.3890 |
frame_dropna_axis1_all                       | 426.2613 | 299.9337 |   1.4212 |
```

#### 2nd run

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
read_parse_dates_iso8601                     |   2.3813 |   2.3716 |   1.0041 |
...
strings_match                                |  12.7503 |  10.5516 |   1.2084 |
timeseries_1min_5min_mean                    |   1.5577 |   1.2853 |   1.2119 |
reindex_daterange_backfill                   |   1.8867 |   1.5500 |   1.2172 |
frame_ctor_dtindex_CBMonthEndx1              |   7.7756 |   6.3493 |   1.2246 |
eval_frame_add_all_threads                   |  35.2084 |  28.6183 |   1.2303 |
frame_ctor_dtindex_CustomBusinessDayx1       |   3.8170 |   3.0283 |   1.2604 |
eval_frame_mult_one_thread                   |  37.0866 |  29.2060 |   1.2698 |
eval_frame_chained_cmp_python_one_thread     | 135.0936 | 104.4240 |   1.2937 |
query_with_boolean_selection                 |  74.9604 |  57.4800 |   1.3041 |
dtindex_from_series_ctor                     |   0.0354 |   0.0270 |   1.3088 |
strings_center                               |   9.6043 |   7.3230 |   1.3115 |
stat_ops_frame_sum_int_axis_1                |   2.3720 |   1.7384 |   1.3645 |
strings_pad                                  |  10.5731 |   7.6413 |   1.3837 |
groupby_multi_different_functions            |  38.6817 |  25.0644 |   1.5433 |
```
 lookw ok to me

cc @ischwabacher 
cc @rockg

can you give a once over pls
 This is kind of difficult to go through.  The tests look okay to me...cannot comment on the specific changes, though.
 ok ping in green
 @jreback Yep, now green.
 thanks!
