* Cast string to sym when checking event in `fire`
* Return early from method if event found
* Change message to reference Event, not State
* Add UndefinedEvent exception
Previously the code raised "UndefinedState" when it failed to find an
Event. This is confusing but was already released to the public. So this
adds UndefinedEvent as a subclass of UndefinedState and raises it
instead.
* Update Changelog