Move test_plots into mpld3 package for use with nosetests

also rename process_testplots.py -> visualize_tests.py

I think this is the testing solution we're looking for.  There is now a ``mpld3/test_plots`` directory, so that test plots are included with nosetests. I also renamed process_testplots.py to visualize_tests.py and modified it so that it would correctly run the test_plots scripts in their new location.

This is the best of both worlds: we have the visual tests, and also the nosetests run all the plotting scripts.