Alter the server event payload abstractions

Unblocks https://github.com/racker/java-service-registry-client/pull/33

* Let objects.Event be a first class object.
* Get rid of event.server.*
* EventPayload becomes a tagging interface.
* Introduce EventType enum.
* Replace event deserialization with a switch/case (this may have been going too far--call me on it).