Kulla API changes: shifting to events

[Robert Field]

With keys generated for RejectedFailed and the addition of Dropped and 
particularly Overwritten Status, Outcome wasn't really making any sense 
any more.  There has been a desire to move in the direction of events.  
The two go well together.  I've introduced StatusTransitionEvent, and 
plan to shift to it.

As soon as I can get the tests converted, I will be using 
StatusTransitionEvent as the event model for update.  I will leave the 
existing enum Outcome and Result.updates in place until we can convert 
everything, but please consider them deprecated.

Please let me know any concerns, questions, ...

-Robert