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`
Move travis uploading back into .travis.yml
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.