BUG: Fixed github stats retrieval

Currently urllib is used to retrieve a json file from Github, however
Github url used is not static url, urllib2 required to retrieve
information from Github.
Bug: alpha parameter was ignored when fill color is #000000
Added check in autoscale_None for completely masked pcolor plots.
Only set `use_2to3` on Python 3.
BUG: fixed shape validation for manual conf intervals in boxplots
TST: fixed new test and added baseline PNG
fixes issue with using transparency for the figure patch and
being able to see shadows of previous draws underneath.

see: http://stackoverflow.com/questions/18944561/matplotlib-pyqt4-transparent-figure

Added some documentation to `paintEvent`
Merge pull request #2449 from tacaswell/qt4_clear_before_draw

Qt4 clear before draw
Merge pull request #2446 from mdboom/old-distribute

Don't set use_2to3 unless we have to.
Install numpy as a setup_requires dependency so pip will install Numpy first
Fix installing numpy as a setup-time dependency
Add tests for installing into a completely clean virtual environment.
add option to delete empty PdfPages when closed

 - PdfPages objects can be told at initialization to delete the pdf file
   when closed, if no page was actually plotted
 - keep the default to not delete empty pdf files so current behavior is
   not changed
 - also some basic tests for it
Python 3 fixes
Updated gitignore to exclude build files from the examples.
clearer comments in test
refactor docstrings of PdfPages
Merge pull request #2455 from pelson/gitignore_update

Gitignore update
Hide the .cached member of Text so it doesn't get documented.
pep8 clean up (file seems to have been created with indentation of 3
spaces!)