Fix iPython Notebook display() to also render Maps

First of all, thank you very much for this project. It is precisely what I need at the moment

Currently the [Map example](https://vincent.readthedocs.org/en/latest/charts_library.html#simple-map) does not work when executed within an iPython Notebook.

This is because the TopoJson and D3 Geo Projection javascript libraries aren't loaded when initialize_notebook() is called. (see [Vega Template](vega_template.html))

After I made these changes the maps worked for me. Obviously the initial download now takes a bit longer. The other option is to make the Geo libraries optional when calling initialize_notebook()?