Simplify interface for replaying events

The Replay() function now generates actual events rather than event IDs.
This lets us drop Get() altogether.

Rationale:
 - simpler
 - gives the implementation a little more leeway behind the scenes
 - Get() didn't allow for failure (eg if an id produced by Replay()
   expired by the time Get() was called, there was no way for Get()
   to fail gracefully).

Hmm. I guess Repository should probably be renamed Replayer now, but...
meh.
(http://golang.org/doc/effective_go.html#interface-names)