add repr for event objects

When an event object gets dumped in a stack trace, because there
is no __repr__ of it, we don't get any details about what it was.
Add a __repr__ to all the event objects so that we can get this
information on a fail.

Change-Id: I9a62a4348871b748c53b7354ad1b821fe987127d
Merge "add repr for event objects"