Alter the server event payload abstractions
* 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).
do not use interned strings in enum fromString method.
Merge pull request #35 from racker/simplify_server_events

Alter the server event payload abstractions