Simplify ack-serializer

[hmpf]

The serializer is now of the form:

{'event': {'timestamp': foo, 'description': 'bar'}, 
 'expiration': xux}

and allows sending in event-type. Make a simpler serializer:

{'timestamp': foo, 'description': 'bar', 'expiration': xux}

Originally posted by @hmpf in #295