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 expose worker creation in `THREE.CTMLoader.prototype.createWorker` to be able to create a worker from an other source. I can use browserify/workerify like this  : 
```
var work = require('webworkify');
THREE.CTMLoader.prototype.createWorker = function () {
	return work(require('path/to/CTMWorker.js'));
}

```