Added IPython Notebook support

I've written a pretty straightforward module for integrating python-nvd3 into the IPython notebook.

First, credit to https://github.com/areski/python-nvd3/pull/18 for some of the basic ideas I'm using, and the discussion generated by that pull request.  My version takes a slightly different approach, bundling everything into a single file.

I also explicitly used some of ipython's hooks to more directly enable inline display of python-nvd3 graphs, and set things up so that a single javascript initialization is used, with automatic checking for initialization before displaying graphs.

Also included a demo .ipynb file, which can be viewed at: http://nbviewer.ipython.org/gist/jdavidheiser/9552624