Add support for Google Protocol Buffers

[craigminihan]

Protocol Buffers is a much more compact and low CPU overhead protocol compared to JSON. Clients could detect the presence of AvanceDB via a signature sniff or a no side effect PUT/POST and cache the support level of the remote. Alternately passing their willingness to use PB to the server via the Accept header would enable PB responses.

To get max value from this client implementers would need to support it. Should be easy to modify Cradle or Nano via a fork and PR.