ENH/CLN: support enhanced timedelta64 operations/conversions
closes .#3577
realted #3009 
- ENH: add top-level `to_timedelta` to convert string/integer to timedeltas
- TST: add pandas/tseries/tests/test_timedeltas.py
- API: add full timedelta parsing and conversion to np.timedelta64[ns]
- CLN: refactored locations of timedeltas to core/tseries/timedeltas (from a series of functions in core/common)
- ENH: support timedelta64[ns] as a serialization type in HDFStore for query and append (GH3577)

@cpcloud @jtratner comments?
 @cpcloud this ended up morphing...but now have `to_timedelta` top-level function....pretty nice if I do say so myself!
 All of the experimentalists using pandas will now be chomping at the bit for `TimedeltaIndex`. You've teased them with a `to_timedelta`!

One does not simply ....

implement `to_timedelta` without `TimedeltaIndex`

:smile: 
 hah...I just returned a Series, easier (and pretty close actually)
 `TimedeltaIndex` is for you!
