Allow concurrent access to Licenses

This is a fix for rometools/rome-modules#24 which means that concurrent parsing of documents contain CC licensed data won't create runaway threads. You can see a test showing the failure in a52d3d5f7186b748dacd4baeac5a7ba62cec8af6 and the some refactoring and then a fix by switching to ConcurrentHashMap at the head of the branch.