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
Using a stricter check to see if Python was installed as a framework. Also, because of the recurring problems with non-framework Pythons, importing the Mac OS X backend will now fail (instead of just issuing a warning) if Python is not installed as a framework.
Closes issue #1593 - namespace error
Closes issue #1595 - sorting on a list of tuples can sometimes cause crashes
BUG: Fix typo in _AnnotationBase

Value of "72" switched to "12" in d5a1a3dd36f7daaf8b7e2ec5a73fa5fd4df3e026
implemented inverse transform for Mollweide axes
Merge pull request #1620 from tonysyu/annotation-bug

Fix bug in _AnnotationBase
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
first pass at getting fixing Issue #1622
added unit test for zorder on hline and vline
removed redundant code

Do not need to explicitly call set_zorder because it gets done
in the call to coll.update.
Fixes #1630.  When callbacks were deleted, the cid map was not correctly updated.
pep8 compliance