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