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
BUG: Fixed object type missmatch in SymLogNorm

Currently SymLogNorm accepts both linthresh and vmin, vmax with their
native dtype, meaning that the contents of one object cannot be updated
with another due to a mismatch of types.
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.