BUG: fix reflected comparison operations for Categorical

By the time the comparison gets dispatched to _cat_compare_op, the first
argument becomes zerodim array and no longer passes isscalar test.

ENH: add pd.lib.item_from_zerodim to extract values from zerodim arrays

ENH: make pd.lib.isscalar detect pd.Period, datetime.date and datetime.time
Merge branch 'add-basic-zerodim-array-support' of https://github.com/immerrr/pandas into immerrr-add-basic-zerodim-array-support