wx backend should not attempt to render jpeg/tiff if PIL is present.

If PIL is present, the wx backend needs to allow the base class to get the
job done for print_jpeg and print_tiff, instead of trying to take over
duties itself.
Fix #1999: dash value of zero causes infinite loop
Fixed hatch clipping.
Merge pull request #1999 from mdboom/dashes-infinite-loop

Setting dashes to (0,0) results in infinite loop for agg backends
Shared colorbar axes working.
added unittest for colorbar creation.
Updated tests and added docstring.
Rebased and updated tests.
@mdbooms suggestion for handling missing result files.
Merge pull request #956 from pelson/multiaxes_colorbar

Shared axes colorbars & finer location control
Don't overwrite unit_circle vertices -- causes problems for many other tests
Merge pull request #6 from mdboom/hatch_clipping_v1.2.x

Don't overwrite unit_circle vertices in clipping test
Fix #2006: stretch is invalid, introduced in cc61700
Allow Paths to be marked as readonly, mark all of the singletons as readonly, and deepcopy those Paths in tests that need to modify the path.
fix #2005: Animated artists should be included when saving to a file
Add docstrings
Use with statement for assert_raises
Fixes a number of bugs introduced by cc61700.  texmanager will now still pass when font.family is a list of strings (though there's no sane way for LaTeX to deal with that, but that's nothing new).
Merge pull request #2006 from mdboom/fix-stretch-invalid

ValueError: stretch is invalid
Make weighting by position in the list less magical