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
Add a link to matplotlib questions on stackoverflow.
Fix formatter reset when twin{x,y}() is called
Fix invalid transformation in InvertedSymmetricalLogTransform.
for interpolation='none', the affine matrix is calculated in the display coordinate
Checkable pan/zoom buttons for QT NavigationToolbar
FIX nose.tools.assert_is is only supported with python2.7
Subplot.twin[xy] returns a Subplot instance
updated api_changes.rst to reflect the return value change of twinx and twiny
Update update API changes

Explicitly document the slight change in behaviour of Axes.twin[xy]
Add note regarding use of LogNorm with hist2d
Add example for logarithmic hist2d
FIX undefined elements were used at several places in the mlab module
Deleted FIXME that had no reason to be
Fix overwrite of the minor formatters and locaters
Add a twin axes test
Add a *simple* arrow example
Merge pull request #1183 from NelleV/mlab_fixes

FIX undefined elements were used at several places in the mlab module
BUG: Fix streamplot when velocity component is exactly zero.
Merge pull request #1169 from leejjoon/fix-subplot-twinxy

Subplot.twin[xy] returns a Subplot instance