IPython notebook integration

Copy of the vincent project strategy to embed nvd3 in current master IPython Notebook.

 - add an initialize_notebook function that loads the js
 - add a display method in the NVD3Chart class
 - add a small example base on the scatterChart example

Need to work on a way to create an unique id for the div. Currently, we need to take care of the name used at the chart initialization...

Any thoughts ?