CTMLoader: get url of CTMWorker on loading CTMLoader.js

Current implementation requires the path to be hard coded in the load() call as worker:"path/to/CTMWorker.js".
Here we detect where CTMLoader.js was included from and we assume CTMWorker.js is in the same folder.

Note that this method of catching the href of the lastest read `<script>` might fail if it's included in otherway ...