add three.js JSON format 3 support

Added import and export for three.js JSON format 3 described in
https://github.com/mrdoob/three.js/wiki/JSON-Model-format-3

Support now only for vertices and faces.  Hopefully in the future more can be supported as a result of #21.  Possible would be material, uvs, vertex uvs, normal, vertex normals, color, vertex colors. and morphtargets.