Compare pep

The matplotlib.testing.compare module was a PEP8 nightmare (3 space indents etc.) which I've seen many a time and never got around to fixing... until now!

I've taken a thorough review of the code, and whilst it isn't particularly pretty, it does do a job, so I've not made any interface modifications, but I have taken the opportunity to add some docstrings and improve some of the error message formatting code (its all superficial and not functional).

Anyway, as a result of this, I now know how the RMS is calculated at least :smile:.