WIP: First stab at a rolling_cov_pairwise function. Fixes GH1853
TST: Added a test for rolling_cov_pairwise similar to the test for
rolling_corr_pairwise.
REF: Factored out commonalities between rolling_cov_pairwise and
rolling_corr_pairwise into one reusable function.
REF: Refactored further in preparation for accomodating ewmcov and
expanding_cov.
TST: Refactored the pairwise moment tests in order to be able to reuse them for
other moments.
TST: Added tests for ewmcov_pairwise and ewmcorr_pairwise.
ENH: Implemented ewmcov_pairwise and ewmcorr_pairwise.
TST: Added unit test for expanding_cov_pairwise based on the test for
expanding_corr_pairwise.
ENH: Implemented expanding_cov_pairwise and refactored the
expanding_corr_pairwise implementation to use _flex_pairwise_moment which gives
access to the other keyword arguments as well as allowing for different columns
in the df1 and df2 arguments.
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.
CLN: Fixed PEP8 violations since there weren't many.
DOC: Updated documentation for pairwise API changes.