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)
Oh, I see, I would still avoid the `THREE.CTMLoader.prototype.createWorker` function though. I don't see why it can't just be inlined.