TST: Added tests for pairwise behaviour in rolling moments.

Added a test for rolling_cov_pairwise similar to the test for
rolling_corr_pairwise.

Added tests for ewmcov_pairwise and ewmcorr_pairwise.

Added test for expanding_cov_pairwise based on the test for
expanding_corr_pairwise.
ENH: Implemented pairwise rolling moment functions.

Implemented rolling_cov_pairwise function.

Implemented ewmcov_pairwise and ewmcorr_pairwise.

Implemented expanding_cov_pairwise.

Refactored the rolling moment functions to use _flex_pairwise_moment.
ENH: Folded the *_pairwise rolling moment functions into the base function API.
DOC: Updated documentation for pairwise API changes.

Added a release note and example usage for pairwise=True.
ENH: Marked rolling_corr_pairwise and expanding_corr_pairwise as deprecated.
Merge pull request #4950 from snth/pairwise

Pairwise versions for rolling_cov, ewmcov and expanding_cov