COMPAT/BUG: compat with pytz 2014.3, exposing several test that don't localize
            properly (GH7137)
Merge pull request #7144 from jreback/tz

COMPAT/BUG: compat with pytz 2014.3, exposing several test that don't localize (GH7137)
add sem to nanops
add tests for nanops
add sem to aggregate methods
add sem to groupby
add sem to docs
simplify groupby's std method