Create a blender collision model exporter

Iterate over all objects in layer 2 (configurable), and output them in json for use as a collision model. Initially just export everything that is a cube (and assume everything is a cube), but later on it'd be cool to export cubes and convex polyhedra, and complain if there are any convex / degenerate objects in the layer. I don't know if there is a standard json format for outputting collision models (the cannon.js bunny example just uses something @schteppe made up), but maybe we could use the three.js json format, if it supports multiple objects. Might be able to use code from the three.js exporter (but with all the material stuff stripped out).