Allow client to return list of slaves

Context: I'm developing an application which reads (never writes) from a redis database. This database has a master and two slaves, and three sentinels to monitor the whole thing. I'm already using this gem to handle failovers, but since we're only reading, I'd like to be able to read from the slaves as well. It does not matter if the masters and the slaves are a bit out of sync in my case.

So I added two methods, `Redis#slaves` and `Redis#all_clients`, with the first one returning only the slaves clients, and the second one returning the slaves + the master clients.

Please let me know if there is anything on your mind regarding this feature, thanks!