GeoJSON representations of GeoDataFrames

I've added a to_json method to GeoDataFrame. Next thing for me is a demo/recipe: figure out how to load a GeoJSON file, join data from another table, write out another GeoJSON file. Classic GIS, but all in Pandas instead of ArcMap or QGIS :)

I couldn't resist regrouping and sorting imports in geodataframe.py: standard libs, then dependencies, then the local modules (Zope habits die hard). Is that okay?