Add getTypeDef for querying the Protobuf class in a ProtobufWritable directly

Hi,

Here is an attempt to add a getTypeDef method for querying the Protobuf class in a ProtobufWritable directly. As I have mentioned in issue #327, with the help of  getTypeDef, we do not need to require users to provide the Protobuf class name if we get the ProtobufWritable.

Any comments would be appreciated.

Thanks