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)
I think I would do something like this instead:

```javascript
var worker = new Worker( ( parameters.workerPath || "js/loaders/ctm/" ) + "CTMWorker.js" );
```