This appears to break compatibility with already-compiled .pb.rb files, which use `include` instead of `extend` for `ProtocolBuffers::Enum`. Forcing everybody to re-compile is bad, I'd prefer a solution that continues to work with `include`. Maybe adopt the common ruby pattern of using the `Enum.included` hook to mix in a `Enum::ClassMethods` module, and then just continue to use `include`?

Some test coverage would be greatly appreciated as well