Redefine match
closes #5075

How does this look? If the docstring and the tests reflect our consensus, I'll take a stab at the docs. This is the gist of it:

Default behavior is unchanged, but issues a warning.

```
In [1]: Series(['aa', 'bb']).str.match('(a)(a)')
pandas/core/strings.py:333: UserWarning: This usage of match will be removed in
            an upcoming version of pandas. Consider using extract instead.
  UserWarning)
Out[1]: 
0    (a, a)
1        []
dtype: object
```

New, more useful behavior is available through `as_indexer`.

```
In [2]: Series(['aa', 'bb']).str.match('(a)(a)', as_indexer=True)
Out[2]: 
0     True
1    False
dtype: bool
```

P.S. There's a stray commit in here. Not sure why 88716e4 got lumped in....
