Add Cluster Support

[tubbo]

Now that the official Redis client has clustering support, there's no
real use for the :avoid_multi_commands option, but it will be kept
around until the next major version. You can now just set your Array of
cluster URLs like so...

cluster: %w[redis://first.host:6379 redis://second.host:6379]

...and the #pipelined method will be used to send commands over to
Redis.

This allows redis-store/redis-rails#93 to get merged.