Add Riak Disk Stats WM API (DO NOT MERGE)

Opening a PR for peer review before a customer demo. 
This adds a Riak erlang HTTP client to CS, and adds a Riak disk stats webmachine API call (for use with Riak CS Control).
Implements issue https://github.com/basho/riak_cs/issues/612 , for use with https://github.com/basho/riak_cs_control/issues/39