Allow requesitng *ack* when event is received

[pipermerriam]

from #89

What was wrong?

It's easy to accidentally broadcast into the void with nobody subscribing to your broadcast and thus nobody listening.

How can it be fixed?

Extend BroadcastConfig to have a mechanism for requesting the receiver provide and ack.

Care should be taken to correctly handle or restrict this API for broadcasts that are sent to multiple recipients. The initial implementation should probably error if the broadcast is not limited to a single endpoint (such as requiring that filter_endpoint be set)

However, it should be reasonable to do something like count how many endpoints the event was broadcast to, and then to wait until we receive the same number of ack responses before returning to the user.

I am inclined to make this the default behavior.