Ipython notebook svg

Hi all,

I'd like to add svg representations of the geometries in shapely and accompanying `_repr_svg_` to `base.BaseGeometry` so that the SVG representations are nicely displayed in iPython notebook. It also gives quick feedback on whetheror not a geometry is valid or invalid by colouring blue and red respectively.

I've uploaded an example notebook that you can view through the iPython notebook viewer [here (http://nbviewer.ipython.org/gist/om-henners/10006338)](http://nbviewer.ipython.org/gist/om-henners/10006338)