MRG: Py3K compatibility!!!

This PR contains the last changes necessary for mpld3 to work with Python 3.

I've tested with Python 2.6, 2.7, 3.3, and 3.4: tests pass and plots work!

I haven't tested with IPython notebook in Python 2.6 (py26 notebook is not in conda) but the notebook works with Python 2.7, 3.3, and 3.4.

In 3.4 matplotlib generates a number of deprecation warnings, but that doesn't affect the javascript output.