ENH: Add regex=True flag to str_contains
Using regex=False can be faster when full regex searching is not needed.
See http://stackoverflow.com/q/20951840/190597

Example use case:

```
import string
import itertools as IT
import numpy as np
from pandas import Series

def make_series(letters, strlen, size):
    return Series(
        np.fromiter(IT.cycle(letters), count=size*strlen, dtype='|S1')
        .view('|S{}'.format(strlen)))

many = make_series('matchthis'+string.uppercase, strlen=19, size=10000) # 31% matches
few = make_series('matchthis'+string.uppercase*42, strlen=19, size=10000) # 1% matches
```

```
In [115]: %timeit many.str.contains('matchthis')
100 loops, best of 3: 4.93 ms per loop

In [116]: %timeit many.str.contains('matchthis', regex=False)
100 loops, best of 3: 2.44 ms per loop

In [118]: %timeit few.str.contains('matchthis')
100 loops, best of 3: 4.81 ms per loop

In [119]: %timeit few.str.contains('matchthis', regex=False)
100 loops, best of 3: 2.37 ms per loop
```

can you add a vbench for both of these? (with and w/o a regex)....

no vbenches for strings at ALL!

pls add `vb_suite/strings.py` and add to `vb_suite/suite.py`

bonus points for adding benches for more string ops (could be another PR if you want)!
 also, pls add a release notes refering this as an API change (you can put in the 0.13.1 bucket)
 Would `is_regex` be a better name for the parameter?
 @cancan101 no `regex` is used in `filter/replace` so consistent
 I added some Benchmarks. Note that I modified `_wrap_result` (commit da8d451) because `x.str.cat` returns a string, and strings have no `ndim` attribute. Without the change `x.str.cat` raises an AttributeError.
 can you post the run of this benchmarks ?
 Running 

`time ./test_perf.sh -b master -t str-contains  -r strings`

yields

```
Invoked with :
--ncalls: 3
--repeats: 3


-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
strings_lower                                |   3.5593 |   3.8073 |   0.9349 |
strings_title                                |   4.1630 |   4.3960 |   0.9470 |
strings_match                                |   5.2484 |   5.4413 |   0.9645 |
series_value_counts_strings                  |   6.8393 |   7.0876 |   0.9650 |
strings_upper                                |   3.9666 |   4.0997 |   0.9675 |
contains_many                                |   4.8403 |   4.9907 |   0.9699 |
strings_rstrip                               |   3.2744 |   3.3757 |   0.9700 |
strings_get                                  |   2.6247 |   2.7027 |   0.9711 |
contains_few                                 |   4.7243 |   4.8621 |   0.9717 |
strings_lstrip                               |   3.2550 |   3.3450 |   0.9731 |
strings_strip                                |   3.5814 |   3.6740 |   0.9748 |
strings_slice                                |   2.5616 |   2.6247 |   0.9760 |
strings_pad                                  |   4.1074 |   4.1869 |   0.9810 |
strings_startswith                           |   3.5427 |   3.6077 |   0.9820 |
strings_endswith                             |   3.5497 |   3.6040 |   0.9849 |
strings_count                                |   5.4746 |   5.5540 |   0.9857 |
strings_len                                  |   2.1100 |   2.1390 |   0.9864 |
strings_center                               |   4.1567 |   4.2037 |   0.9888 |
strings_repeat                               |   4.1867 |   4.2174 |   0.9927 |
strings_findall                              |   8.4347 |   8.4640 |   0.9965 |
match_strings                                |   0.4900 |   0.4887 |   1.0026 |
strings_extract                              | 570.6987 | 568.8750 |   1.0032 |
strings_replace                              |  12.6190 |  12.5709 |   1.0038 |
strings_join_split                           |  31.8976 |  31.4066 |   1.0156 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [b2bdbf5] : PERF: add benchmarks for every str method
Base   [0bab303] : BLD: python3 unicode issue in print_versions


```
 @unutbu grt! hmm.....maybe have a look at the last 3 to see if anything obvious can do? (esp extract)?
 @unutbu can you squash to a smaller num of commits....

@y-p @jtratner look ok?
 Where's the perf difference? all those benchmarks look like 1.0x to me, or is that 4% _it_?
 @y-p: There should be no performance gain shown in the Benchmarks (because the default behavior, `regex=True`, was left unchanged). The `regex=False` keyword in `x.str.contains(pat, regex=False)` can result in a 2x perf gain for users when the pattern is a plain string however. See http://stackoverflow.com/q/20951840/190597 for a use case.
 @unutbu I think that you show these benchmarks, but they are not appearing in the above list; this is a case where you compare across benchmarks
e.g. maybe paste the result of these (can just be in ipython as well - in the top section would be great)

```
strings_contains_many = Benchmark("many.str.contains('matchthis')", setup)
strings_contains_few = Benchmark("few.str.contains('matchthis')", setup)
strings_contains_many_noregex = Benchmark(
     "many.str.contains('matchthis', regex=False)", setup)
strings_contains_few_noregex = Benchmark(
     "few.str.contains('matchthis',
```
 >  can you add a vbench for both of these? (with and w/o a regex)....

No matter, I tried this here and it does indeed give a nice boost.

+1
 @jreback: I'd be glad to but I don't know how.

When I run `./test_perf.sh -b master -t str-contains -r strings`

those Benchmarks appear to be missing. And some others (with no `strings` in its
name), like `contains_many` and `contains_few`, are present.

I still don't understand how to use vbench very well. Is there any documentation?
I would like to learn how to run benchmarks from IPython.
 @unutbu that looks right....

try `test.perf.sh -H -r strings`. I have always found it has issues when the benchmarks themselvs are new

I just meant to do `timeite` from IPython....for a quick comparison of old/new that is ok (for display purposes in a PR).
 The filtering is on the benchmark name (first arg of `Benchmark()`) not the name of the python variable.
 if regex is False but the pattern is actually a regex should this raise? (not sure if its easy to determine if its an actual regex)
 any regex is also a valid string, so you can't disambiguate.
 Someone might have a list of regex strings and wish to find those that contain a literal string like `(.*)`. So the pattern could look like regex and yet one might still wish to search for it like a plain string. Maybe that's a stretch, but it seems conceivable. :)

@jreback: I tried `test_perf.sh -H -r strings` and it did find and run the missing benchmarks, but with no comparison to master (of course). I then reran `./test_perf.sh -b master -t str-contains -r strings` but there was no change in the benchmarks found.
 a real world example:

'Mr. Smith'

Do I want to match Mr./Mrs smith or an exact match?
 yep...ok
 @y-p: I think filtering is being done in test_perf.py with

```
benchmarks = [x for x in benchmarks if re.search(args.regex,x.name)]
```

and `x.name` is being set in `vbench/benchmark.py` by the `_get_assigned_name` function, which ultimately does this:

```
varname = line.split('=', 1)[0].strip()
return varname
```

Moreover,

```
from vbench.api import Benchmark

foo = Benchmark('bar','baz')
print(foo.name)
```

prints `foo`, so it looks like the `Benchmark().name` attribute is being set to the variable name. 

But I must be missing something because I don't know why some Benchmarks are not being run. 

Maybe this is a clue: `contains_many` and `contains_few` were old variable names for Benchmarks I made in `vb_suite/strings.py`. They no longer exist in the `str-contains` commit, nor in `master`. They do exist in `vb_suite/benchmark.db`, though. For some reason these benchmarks are getting called when they should not even exist...
 That explaines why I got different results from you, erase the db and they'll probably vanish.

I had no idea wes used the `inspect` trick. Usually frowned upon as too much magic.
https://www.youtube.com/watch?v=8e0l_Dt28MQ

Update: I'm trying to do too many things at once, Of course the first arg is not the name.
 @unutbu thanks for the PR....nice work!
