Add Serialization for protobuf enum values

[tomjack]

This adds another Serialization impl, for the actual enum values (not the enum value descriptors, like ProtobufEnumSerialization).

I used reflection to get at the enum classes "valueOf". Since I only get the method once, I hope it does not add terrible overhead.

By the way, ProtobufEnumSerialization seems incomplete — deserialize returns null. Do we also want serialization for the enum value descriptors? I guess the motivation was that getField for an enum field returns a value descriptor instead of the actual value?

[bryanduxbury]

You are correct about why there's no enum value serialization - getField for an enum returns a descriptor and not a type. I'm not averse to adding a serializer for enum values... but how would one go about extracting such a value? The only way I can think of right now is through a custom function.

In any case, I would like to see a unit test that covers this new code before merging.

Also, it looks like the existing enum deserializer just plain wasn't implemented all the way. That needs to be remedied.