CLN: Add axis kwarg to Series wrapper for compat

Update tests to cover full range of Series flex ops and check that they
all accept an axis argument.
Merge pull request #5352 from jtratner/add-axis-kwarg-to-series-flex

CLN: Add axis kwarg to Series wrapper for compat
BUG/ENH: Allow drop to accept axis name

Add tests for DataFrame, Series and Panel, with axis checks, etc.
Merge pull request #5354 from jtratner/allow-drop-to-accept-names

BUG/ENH: Allow drop to accept axis name
DOC: Added/fixed some docstrings, esp Panel flex

str representation of functions is not pretty.

DOC/CLN: Wrap 0.13's release.rst at 80 characters per line.
Merge pull request #5336 from jtratner/minor-doc-cleanups

DOC: Added/fixed some docstrings, esp Panel flex