BUG: Fixed bug in _flex_binary_moment and PEP8 violations in vicinity.
REF: Refactored ewmcov and ewmcorr to use _flex_binary_moment.

This brings them in line with like rolling_cov and rolling_corr so that
the pairwise behaviour we're going to add will work there too.
ENH: Added ability to calc pairwise cov to rolling_cov API.
ENH: Rolled out pairwise behaviour to the other rolling functions.