Three based viewer component

This is working against the Physics2 scene (modded to have json versions of models).
To test, start server with --file http://playsign.tklapp.com:8000/Physics2-WebTundra/scene.txml and open app.html from here  (or browse to http://playsign.tklapp.com:8000/Physics2-WebTundra/app.html)

Next steps with this will probably be to switch to three scene nodes (Object3D) per Placeable (instead of the hardcoded mesh+placeable -> naked three Mesh nodes now).