CLN: more plotting test cleanups
Just a couple of cleanups here and there

Adds a new decorator to skip matplotlib in a class method

@jreback merge? just a couple of clean ups regarding testing and added a decorator to inject a matplotlib test skip for `unittest.TestCase` classes.
 this looked ok 
 FYI, seeing `ImportError: No module named nose` in clean virtualenvs (w/o nose) after the util/testing.py change in this PR was merged
 yes ... better to just remove debug from `pandas.__init__.py`
 and i'll move `nose` out of the toplevel import
