Add Yokozuna Admin messages.

To satisfy [administrative support in PB for yokozuna](https://github.com/basho/yokozuna/pull/144), this adds 7 new messages:

* Index GET req and resp
* Index PUT req (resp is standard put response)
* Index DELETE
* Schema GET req and resp
* Schema PUT req (resp is standard put response)

Currently yokozuna piggy-backs on Riak Search query messages for queries,
and this will continue. The new riak_yokozuna messages are only administrative