Support Redis auth

[analytically]

host, port, password

http://redis.io/commands/AUTH

[azylman]

The best way to do this is probably to change Clever/leakybucket to use soveran/redisurl to dial in the redigo connection pool: https://github.com/Clever/leakybucket/blob/master/redis/redis.go#L143-145.

We'll probably get to this at some point, but if you want it soon, it should be a pretty easy PR. It might need a new New method with a different signature for backwards compatibility, but I think this can actually be a drop-in replacement for the existing Dial method we use.