Prune the subtree from search for @ThriftService annotations when one is found

If you have class A implements interface B, which extends from interface C, and class A is NOT annotated with @ThriftService but interface B and C are each annotated with @ThriftService, this should be fine and not produce a conflict, because interface B's @ThriftService should hide the one from interface C.
add tests
Merge pull request #75 from andrewcox/fix-find-thrift-service-annotation

Prune the subtree from search for @ThriftService annotations when one is...