code: def subscribe(channel, proc = nil, &block)
        if cb = proc || block
          @sub_callbacks[channel] << cb
        end
        @subs << channel
        raw_send_command(:subscribe, [channel])
        return pubsub_deferrable(channel)
      end
docs: Subscribe to a pubsub channel
  
   If an optional proc / block is provided then it will be called when a
   message is received on this channel
  
   @return [Deferrable] Redis subscribe call