Add counters to riakc_pb driver

Ability to bench counters.

Note: counter buckets _MUST_ be `allow_mult=true`. In this case I would set a new bucket, with `riakc_pb_bucket` in the config, since using the default `<<"test">>` bucket will cause conflicts with other pb test runs.

Also, requires the added `random_int` val gen, so can't be mixed with other pb operations at the moment, though maybe I'll change that, if the reviewer thinks it is best just to use random in `run/4`