OneOf Support

[kiwidoggie]

I was testing this on a game's save data. I noticed that oneof doesn't seem to be supported properly. In the protobuf source, it seems that inside of FieldDescriptorProto the oneof_index is case (9): I found this by looking through the descriptors binary data for 480142150A which the first byte 48 >> 3 = 9 At the current time of manually by-hand decoding, I have been setting just the optional field (which works for rebuilding) but not entirely sure that is correct.