Make Axes.stem take at least one argument.

When it takes only one, the abscissae default to np.arange(len(y))
Add axes.stem call signature test
Update CHANGELOG with axes.stem change
Fix positional argument parsing
Make stem catch explicit Exception classes
Document the C/C++ code guidelines
implemented inverse transform for Mollweide axes
added unit tests for Mollweide transforms

The new test functions test_mollweide_forward_inverse_closure and test_mollweide_inverse_forward_closure test that the forward->inverse and inverse->forward compositions are approximate identities.
use ax.transProjection instead of ax.transData in Mollweide axes unit test

The goal was to exercise the MollweideAxes.MollweideTransform and MollweideAxes.InvertedMollweideTransform classes. The ax.transData property does not make use of either of these classes; the ax.transProjection property does.
reduce precision to the level at which the test passes
Merge pull request #1606 from mdboom/cxx_extension_policy

Document the C/C++ code guidelines
Initial work on WebAgg backend
mplot3d should call canvas.draw_idle()
Address documentation and smaller issues raised by @pelson in #1426.
Revert change to how width and height are handled -- the other was causing image resampling.
Handle slightly older versions of Firefox.
Handle non-binary image streams
Display URL if not set to open webbrowser
Remove some logging
Fix toolbar button events on Firefox.