add qos support

Hi! I've added qos support. I've only checked prefetch_count, so I don't guarantee that other parameters work.
I see. The problem is that `all_channels` is a misleading name because, depending on the AMQP broker implementation, it might not in fact affect all channels. (If this function wasn't part of the public API I wouldn't mind!)

If possible, I'd like to come up with a name that, while being clear, doesn't rule out either definition. If we can't think of a good one, I think we should go with the RabbitMQ definition, since it's by far the most popular implementation.

How about `apply_globally`?