Riak 1.4 Feature: Counters

Riak 1.4 introduces convergent PN-counters based on the prototype work
done in [riak_dt](//github.com/basho/riak_dt).

Implementation details:
* Counters can only be used on buckets with `allow_mult` set to `true`. This should be guarded in the client.
* Increment requests should _not_ be retried if there was a timeout or quorum failure.
* Increment requests allow most PUT options.
* Value requests allow most GET options.
* Only positive and negative integers should be allowed for increments.
* Increment requests have a `returnvalue` option which will return the resulting counter value after update.