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)
But this would require users to specify the path at each load() invocation. The THREE.CTMLoader.workerURL attribute allows to override the URL once and for all subsequent calls