patch to fix issue #2473.

The change to the agg propagate to how `imsave` works so one test had
to be changed to account for the fact that pixels with alpha=0 are now
set to (1, 1, 1, 0) instead of (0, 0, 0, 0).
added class variable `_fill_color` to RendererAgg to hold the
color to use in `clear` calls to the render_base.

Changed all uses of `clear((...))` -> `clear(_fil_color)`.
changes to c++ because gcc 4.8 seems to allow things the compiler
on travis will not.
Fix #2472: Draw the spines on top of twin axes
Merge pull request #2502 from pelson/gca_doc

Updated the docs of pyplot.gca.
minor docstring fix in dates.py
tiny correction of matplotlibrc.template

remove quotes around text.hinting default value (i.e. auto)
Merge pull request #2472 from mdboom/twin-spines-on-top

Plots using twinx draw on top of figure frame
Merge pull request #2479 from tacaswell/rastized_background

Set default agg canvas background color to ```(1, 1, 1, 0)```, rather than ```(0, 0, 0, 0)```.
Update version to 1.3.1
Fixed bad quiver transforms.
Fixed failing test on travis.