Refactored SceneLoader.

Just a single callback is now passed  to loader.load( "scene.js", callbackFinal ) - callback which is called when everything from the scene was already loaded.

If you want to use also callbacks for initialization of procedural synchronously created elements, you can set property of loader:

loader.callbackSync = function( result ) { ... }

If you want to get callback for load progress, set:

loader.callbackProgress = function( progress, result ) { ... }

Also added per scene creation of collision systems (it used to be singleton).