Writing event listeners

TODO: write about event listeners in general

Our event system is driven by RabbitMQ, most of which you can ignore, because we've simplified it down to two kinds of event listeners:

Basically you either get: * Immediate, best effort messages * Queue based messages will be processed (eventually)

The pyforge platform creates a pool of queue consumers that handle messages, and it calls all the Reactors that are registered for that event.

Or, you can ask for immediate message receipt, with now guarantee of delivery.