Experimental: Interactive, zoomable feature filtering widget

This is the feature that I mentioned in #108. [Here is a demonstration.](http://nbviewer.ipython.org/gist/anonymous/58bdde32f7490845f6af) You will need to run it locally to see the widget. It's pretty cool.

I pushed this to a new branch of soft-matter/trackpy so it will be easy for others to find. We can remove that once we decide to merge it into master. I imagine that this `trackpy.interactive` module will never be auto-imported; it will be exclusively for use in the notebook.