Version 1.3.1

[Jeff Alstott]

Updated to version 1.3.1. Check it out on PyPI or Github.

This was primarily a bug fix release:

- Matplotlib had changed some of their defaults underneath us. Now pdf plots that have empty bins should render correctly.

- Should now actually work with Python 3, as opposed to load in fine but promptly break if you tried actually do any fitting with it.

- Also souped up the demonstration IPython Notebook a bit.