BUG: Bug in idxmin/idxmax of ``datetime64[ns]`` Series with ``NaT`` (GH2982_)
     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)
     minor doc update in io.rst (pytables)