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....