REL: v2.1.1

The first and only planned bug-fix release for the 2.1 series.

This release include many bug and documentation fixes.  Highlights are:

 - change default logscale behavior to clip
 - fixed webagg import errors
 - tweaks to polar ticks
 - fixed exception when guessing afm family names
 - update manifest to include all doc files
 - improve masked array handling in pcolormesh
 - fix segfault from reading invalid png
 - fix blocking_input
 - fix bug resulting in huge multi-page pdf files
 - make `show` in nbagg take args as other backends do
 - several fixes to OSX backend
 - restore positional arg handling on Figure.legend
 - fix bugs in interactive navigation
 - fix issues when panning with invalid limits
 - fix bug with fully masked arrays passed to imshow

Author: tacaswell