Merge remote-tracking branch 'upstream/v1.3.x'

Conflicts:
	lib/matplotlib/tests/test_backend_pdf.py
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
Skip CGContextSelectFont if font lookup fails
Merge pull request #2433 from mdboom/unicode-font-filenames

Handle Unicode font filenames correctly/Fix crashing MacOSX backend
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!)
a bit more pep8 work
Add API change note.
Merge pull request #2457 from mdboom/private-members

Privatize Text.cached
Fix building of tarball