Split 'ack' into done/fail/progress

[rykov]

To model after jQuery.Deferred(), I would prefer a per-message response that indicates more than just an acknowledgement of receipt. Just like jQuery.Deffered() has callbacks for done, fail, and progress, I think it would be useful to have those handlers. At the moment, it's impossible to support per-message progress that with 'ack' because the callback handler is removed on the first 'ack' and ignores subsequent ones.

What do you think of this idea? Is there any way to support this with the current Javascript client?