Json enum names functionality

This https://github.com/libgdx/libgdx/commit/1848a47c8c9553d9954dc7326433e53a4a2e3040 commit added Json#enumNames attribute but I didn't see any implementation related so I've decided to add it along with a test.

@NathanSweet could have a look at the first commit to make sure everything is fine.