BUG: Bug in idxmin/idxmax of ``datetime64[ns]`` Series with ``NaT`` (GH2982)

fixes GH #2982

- Bug in idxmin/idxmax of ``datetime64[ns]`` Series with ``NaT``
- refactor in core/nanops.py to generically handle ops with
    datetime64[ns]/timedelta64[ns]
- Series.argsort to always return int64 dtype (and ignore numpy dtype on ints),
  was failing a test, indicated in a711db075d5835f45f04867752f3db64be359490
- minor doc update in io.rst (pytables)