Generate Python from message code mappings.

This adds two `setup.py` commands, `build_messages` and `clean_messages`. The former generates `riak_pb/messages.py` from `src/riak_pb_messages.csv`, the latter removes the generated file. `build_messages` was designed to generate a file identical to the one that ships with the current Python client, with the exception that new message codes will generate the correct entries automatically.

Related to #63.