Add Protobuf support to Yokozuna

Protocol buffers are a lower bandwidth alternative to the HTTP interface, and would be nice to implement any future clients (#37) against.

Currently this issue's aim is to match the functionality of RiakSearch. RS only supports a limited number of query params via PB, so this will mainly be of interest for existing RS users wishing to transition to Yokozuna. Also, RS only supports querying, and does not have PB support for creating/managing indexes.