Using classic "loader.load( url, callback )" API instead in OBJLoader, STLLoader, VTKLoader.

OBJMTLLoader is still having just the incorrect EventDispatcher API, but there the situation is more complicated.

Added VTK and STL models to scene loader example. Added optional callback parameter to VTKLoader and STLLoader load to be compatible with other loaders.

Work in progress. Not really sure how to best deal with different loaders using different APIs. We should unify them somehow. Still need to handle ColladaLoader, UTF8Loader and OBJMTLLoader, every one of them is different from the others.