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.