Merge pull request #2976 from jreback/pytables_select

BUG: HDFStore didn't implement != correctly for string columns query, GH #2973
TST: pathological objects cause infinite loop when passed to pprint_thing
BUG: pathological objects cause infinite loop when passed to pprint_thing
BUG: Series.argsort failing on datetime64[ns] when NaT present, GH #2967