BUG: fix Categorical comparison to work with datetime

pd.Timestamp is one of types that don't pass np.isscalar test, that was
previously used in _cat_compare_op.  pd.lib.isscalar should be used
instead.
Merge branch 'fix-datetime-categorical-comparison' of https://github.com/immerrr/pandas into immerrr-fix-datetime-categorical-comparison

Conflicts:
	doc/source/whatsnew/v0.15.1.txt