Add a print format for RiakBucket.
Add a print format for RiakBucket.

GabrielNicolasAvellaneda/riak-python-client,basho/riak-python-client,basho/riak-python-client,GabrielNicolasAvellaneda/riak-python-client,bmess/riak-python-client,basho/riak-python-client,bmess/riak-python-client