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)
Would it be a pain to create the worker by hand?

```javascript
var worker = new Worker( 'js/loaders/ctm/CTMWorker.js' );
var loader  = new THREE.CTMLoader();
loade.load( 'blah.ctm', onLoad, { worker: worker } );
```