Update message codes to support CS, counters and Yokozuna.

Reference: https://github.com/basho/riak-ruby-client/blob/master/lib/riak/client/beefcake/message_codes.rb
Add support for counters
Add deletion of counters.

I'm still waiting on clarification from Basho on how this converges.

As the Riak counter is implemented as PN counter per vnode I'm assuming that it needs an acknowledgement from each primary to reliably converge. I have set the default quorum values accordingly. They can be overridden via options if the developer knows better.

Reference for quorum symbolic integer values: https://github.com/basho/riak-ruby-client/blob/0728c53ea1d86f6e85052f88a90d5da9b766a5dc/lib/riak/client/protobuffs_backend.rb#L104-L110

Reference for PN counter implementation: https://github.com/basho/riak/issues/354
Remove duplicate test helper
Merge branch 'bucket-last-write-wins' into counters
Remove unused imports in counter_test.go
Merge pull request #56 from jcoene/counters

Add support for counters