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)
Still not super convinced... 

What about doing `THREE.CTMLoader.prototype.worker`? That way you could do... 

```javascript
THREE.CTMLoader.prototype.worker = work(require('path/to/CTMWorker.js'));
```

And then... 

```javascript
var worker = scope.worker || new Worker( "js/loaders/ctm/CTMWorker.js" );
```