Add Protobuf::Rpc::Stats.statsd_client API to forward to Statsd

This provides for easy stats collection via a Statsd server.
Add rpc prefix to statsd-client path
Refactor new statsd_base_path out of call_statsd_client

And respond to feedback from rtyler
Merge pull request #6 from dgolombek/wip/statsd_client

Add Protobuf::Rpc::Stats.statsd_client API to forward to Statsd