Adding a geojson plugin

Here is what I came up with for a geoJSON plugin.  It is split into two pieces.  The first is a girder independent module that builds geoJSON objects out of arrays of data objects (such as mongo queries).  The second is a single endpoint under a geojson resource that assembles point feature collections out of an item query.  

I tried to design the interface in a way that it will be easy to add other kinds of geometries if the need arises.    Also, I think it would be interesting to try to add a client side plugin that displays items on a map.