testing: give more informative error when nose plugin not installed
testing: clip abs diff image
testing: convert function names to MPL coding standard
testing: initial spine test (test for SF#2852168)
testing: add new, simplified testing infrastructure
testing: move image comparison functions into matplotlib.testing
testing: workaround issue with nose testing's ZeroNinePlugin
testing: install KnownFailure nose plugin if using setuptools
testing: switch known failure condition to 'indeterminate' for SF bug 2850075
testing: allow fail_condition to be 'indeterminate' for knownfailureif decorator
