Change the IPython Notebook integration

This improves `_repr_html_` in `visualization.py` (I think) and adapts the two IPython Notebook example files to the new capabilities.

I've also simplified the `initialize_notebook` function in `core.py` (I think). Ultimately, imho, it would be cool to make this function an IPython magic that not only does the initialization but also allows the specification of the displayed output to be either SVG, Canvas and maybe even WebGL. So it would be very similar to the `%matplotlib` magic.

    %matplotlib inline

could become

    %vincent canvas

but that's in the future.