DOC : added warning to doc of get_window_extent

This is in response to issues raised in #2831.

Closes #3055
Much better alpha control in mplot3d. Closes #1541

Probably haven't solved them all, but this gets most of them.
ENH: Added depthshade argument for axes3d scatter
DOC : added crucial 'un' to docstring
Merge pull request #3184 from tacaswell/get_window_extent_doc

DOC : added warning to doc of get_window_extent
Fix formatting/linking problems with plt.annotate
at the suggestion of @tacaswell, I'm just adding this auto-generated macosx.py file to the ignore list with it's compadres.
add reduce to the list of imports from six.moves
Merge pull request #3211 from WeatherGod/mplot3d/depthshade

Mplot3d/depthshade
Merge pull request #3221 from katyhuff/mac_pep

Fixes #3219 by ignoring pep8 noncomplicant auto-generated file.
Merge pull request #3126 from mdboom/doc-formatting-junk

insertion of Annotation class docs into annotate docstring broken
Merge pull request #3222 from WeatherGod/mplot3d/py3k_fixes

add reduce to the list of imports from six.moves
DOC : added missing whats_new/api_changes entries