Extract GeoJSON stuff into an extension

[mourner]

It would be nice to have a separate supported RTree implementation without the GeoJSON stuff that can be used for general use cases, and additionally a rtree-geojson.js extension or something like this.

I don't see any reason to have them combined if this is going to become a standard RTree JS implementation.

[calvinmetcalf]

now that I'm using grunt that shouldn't be to big a deal