Riak registry adapter

This adds a dependency on the "riak-client" gem (c.f. https://github.com/basho/riak-ruby-client ).

It uses Riak buckets prefixed as "dcell_environment_" by default, but you can override that. All the options that aren't environment and prefix are passed right to the Riak client, so you can set it up with multiple nodes, protocol buffers, etc.

I know you're changing the API soon, but I wanted to do this for reasons :tongue: