Initial rework of gen_gallery.py

* Creates a toc at the top of the gallery page
* entries are created by a list of sub-directories
* examples from each sub-dir is sectioned
suppress exception upon quitting with qt4agg on osx
Add a link to matplotlib questions on stackoverflow.
Add What's New section for Python 3.x support.  Address #1087 in part.
Fix formatter reset when twin{x,y}() is called
Fix invalid transformation in InvertedSymmetricalLogTransform.
Clarify which backends do and don't work.
for interpolation='none', the affine matrix is calculated in the display coordinate
Add Phil Elson's percentage histogram example
Checkable pan/zoom buttons for QT NavigationToolbar
workaround for fixed dpi assumption in adjust_bbox_pdf
Install six as a proper top-level package.  Fix some other minor and related issues related to installing dateutil and pytz on Python 3.
Merge pull request #1142 from mdboom/whats_new_python3

What's New: Python 3 paragraph
Merge pull request #1155 from pwuertz/temp-tight-bbox-fix

workaround for fixed dpi assumption in adjust_bbox_pdf
FIX - typo in lib/matplotlib/testing/compare.py
Merge pull request #1158 from NelleV/FIX_tests

FIX - typo in lib/matplotlib/testing/compare.py
Add a percent symbol to tick labels

Got to be careful when latex is involved.
Merge pull request #1149 from dmcdougall/percent

Add Phil Elson's percentage histogram example
FIX assert_raises cannot be called with with

  - Fixed 5 tests failing
  - PEP8 compliance
Merge pull request #1154 from mdboom/fix_six

six inclusion for dateutil on py3 doesn't work