Merge branch 'v1.3.x'

* v1.3.x:
  Document API change in hist
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
Updated the docs of pyplot.gca.
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
Merge branch 'master' of github.com:matplotlib/matplotlib

* 'master' of github.com:matplotlib/matplotlib: (90 commits)
  added information regarding new spectrum functions to CHANGELOG, whats_new, and api_changes
  add demo for new spectral plot types
  add tests for new spectrum types in _axes
  add more spectral plots and add new spectrum types to specgram in _axes
  implement tests for refactored mlab spectrum, windowing, and detrend functions, with 100% coverage
  refactor mlab spectrum, window, and detrend functions to improve modularity, testability, and performance
  add CleanupTestCase, a unittest.TestCase with cleanup
  add mlab tests for new spectrum types, window and detrend functions, and greatly expand existing tests
  add more spectrum functions and add new spectrum types to specgram in mlab.py, and clean up window and detrend functions in mlab.py
  Fix for compiling ttconv in extern directory
  cast argv to unicode before testing
  Move external libraries to 'extern' directory
  Minor doc fixes
  Fix encoding issues when font name contains non-ascii characters
  Fix writing to all manner of StringIO-like objects in PS backend.
  add new ps backend test file in `__init__.py`
  BUG: Fixed object type missmatch in SymLogNorm
  add failing test: savefig(StringIO(), format="ps")
  Adding a missing 'b' back into two 'bbox_' kwargs
  Fix build
  ...
Merge branch 'master' of github.com:matplotlib/matplotlib

* 'master' of github.com:matplotlib/matplotlib:
  removing items from the call to six.iteritems
  fix removed api change regarding spectral functions
  - pep8  - removed mutable default values  - foo == None -> foo is None
  pep8
Merge remote branch 'upstream/v1.3.x'

* upstream/v1.3.x:
  Update version to 1.3.1
  tiny correction of matplotlibrc.template
  minor docstring fix in dates.py
  Updated the docs of pyplot.gca.
  Fix #2472: Draw the spines on top of twin axes
  changes to c++ because gcc 4.8 seems to allow things the compiler on travis will not.
  added class variable `_fill_color` to RendererAgg to hold the color to use in `clear` calls to the render_base.
  patch to fix issue #2473.

Conflicts:
	lib/matplotlib/__init__.py
	lib/matplotlib/axes/_axes.py