Generate Python from message code mappings.

This adds two distutils/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.
Remove generation of empty responses, since it is unused.
Merge pull request #77 from basho/feature/sdc/python-msgcodegen

Generate Python from message code mappings.