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.
added percentage rank to DataFrame.Rank
Merge pull request #4950 from snth/pairwise

Pairwise versions for rolling_cov, ewmcov and expanding_cov
Merge pull request #6728 from MichaelWS/master

added percentage rank to DataFrame.Rank
SQL: add index_label keyword to to_sql
Merge pull request #6642 from jorisvandenbossche/sql-multiindex

SQL: add index_label keyword to to_sql
BUG: bug in BlockManager._get_numeric_data, with invalid combine
Merge pull request #6730 from jreback/numeric

BUG: bug in BlockManager._get_numeric_data, with invalid combine
BUG: Bug in resample when how=None resample freq is the same as the axis frequency (GH5955)
Merge pull request #6731 from jreback/resample

BUG: Bug in resample when how=None resample freq is the same as the axis freq (GH5955)
BUG: Bug in downcasting inference with empty arrays (GH6733)
Merge pull request #6737 from jreback/infer

BUG: Bug in downcasting inference with empty arrays (GH6733)
DOC: small doc fixes (build warning in sym_diff + formatting in release notes)
Merge pull request #6739 from jorisvandenbossche/doc-fixes

DOC: small doc fixes (build warning in sym_diff + formatting in release notes)
BUG: MPL Plot cannot make loglog option worked
DOC: minor documenation changes