use ThreadLocal Context Class Loader for loading UIMA resources

A user has reported getting not-found when trying to load resources in a Tomcat environment, while the same application works fine when run as a separate application.

Change the logic for finding UIMA resources via class loaders to consider, if a UIMA Extension class loader is not specified, the Thread Local Context Class Loader. If that is used, and the resource is not found, for backwards compatibility, try to find using the system class loader.