CTMLoader improvements

- allow to set worker url via THREE.CTMLoader.workerURL (usefull when file hierarchy does not match three.js hierarchy)
- allow to delegate worker creation (loader parameters and THREE.CTMLoader.prorotype.createWorker)
Yes, of course, but for performance reasons I need to have several concurrent workers, to load several meshes simultaneously, this is why I need to handle worker creation outside THREE. Performance is even better if I reuse the workers, but I do not see how to do it differently...