Maven will autogenerate RiakMessageCodes

Uses a template and generates com.basho.riak.protobuf.RiakMessageCodes
which will now be packaged along with the protobufs

To verify, run: `mvn clean install` then inspect `target/generated-sources/protobuf/com/basho/riak/protobuf/RiakMessageCodes.java` as well as check the jar: `jar tvf target/riak-pb-2.0.0.12-SNAPSHOT.jar |grep MessageCod`