Interactive feature demo

As part of #92, I thought we needed a nice looking demo application.  This PR adds an example that we could use as a central repository for showing off new features and styles.  For the moment, this example only creates svg features, but I'm using the feature API so it will be easy to add options for the gl renderer as well.

For now, I put the demo up here: http://opengeoscience.github.io/geojs/examples/features.html