Geo interface

Closes #113 , Implements a `__geo_interface__` property on `GeoDataFrames` and `GeoSeries`. Both are represented as geojson-like dicts with type = `FeatureCollection` ... this is inconsistent with the [current spec](https://gist.github.com/sgillies/2217756) which expects features or geometries. As discussed with @sgillies , this is probably an acceptable extension to the interface (which should be updated accordingly)