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
```