Allow waiting for all subscribers to finish

I'm not sure if this is the right way to do it – I read the code dealing with `:block => true` and saw that it's implemented this way, so I figured it would work.

Basically, I want to have a script that sets up a number of subscriptions and just blocks. Probably a signal handler would trap INT signals and close the channel, in which case the `wait_for_subscribers` call would return (unless there's something I've misunderstood).

Does this make sense to add, or is there a better way to accomplish what I'm trying to do? And is the method name right?