API: expose to_numeric/to_datetime/to_timedelta methods on Series
if we have a string Series, e.g. ``s = Series(['2013-01-02', '2013-1-03'])``

we normally just run this thru ``pd.to_datetime(s)``

it might be convenient to do these operations (``.to_numeric,.to_datetime,.to_timedelta``)
directly as methods.

- ``s.str.to_numeric(....)`` , IOW in the ``.str`` namespace
- ``s.to_numeric(...)``, in main Series namespace
- ``s.astype(.., dtype=...)`` but actually pass thru to using the ``.to_numeric/.to_datetime`` functions directly, so you could have something like
  ``s.astype('datetime', format='%Y-%m-%d')`` and it would do the right thing. This overloads things a bit though

right now we have a bit of confusion because we support ``.astype`` which is pretty much a pass thru to numpy, and the more dtype-aware (and functional) ``.to_*`` ones.

Furthermore I think we could update the ``.astype`` docs (and the ``.to_*``) ones to cross reference.