Possibility to retrieve produced event without registering

Producers in Otto are really useful to push states to any part of the code that needs it.
But sometimes, we just need this state once for any reason (save the state, display a notification, ...).
Actually, we must register(), wait for the event and unregister().

This is a simple method Bus.produce(type) that will return the current value from the producer for this event immediately.

This is my very first pull request so I hope I have all done right :).