Add HyperLogLog commands

These PF commands are currently passed along to Redis unmodified. This fixes that.

See: https://github.com/redis/redis-rb/pull/432
While redis-rb only accepts one argument in 3.1.0, that was an oversight on my part. I'll be patching it to handle multiple args in 3.1.1 so this should be `"pfcount" => [ :all ]`.

I'll tackle merging this feature after I get a 3.1.1 out on redis/redis-rb.

Sigh.