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)