Integrate RAT into Gradle build.

There is a tool called RAT (http://creadur.apache.org/rat) which checks the codebase for licenses, mainly looking at files to ensure the license headers are in place.

We should have that being part of the Release task in the Gradle build. Currently, RAT exists for Maven, Ant and command-line, so perhaps we should even contribute something back to Creadur if we get this working.