Change over to use message codes from riak-pb

With the release of riak-pb 2.0.0.14 the protobuf message
codes are automatically generated and included in a class
in that .jar. This deletes the local RiakMessageCodes.java and
changes everything over to use the class in riak-pb

Resolves #415
Merge pull request #421 from basho/gh415-use-generated-message-codes

Gh415 use generated message codes