Add InvalidEvent error class.
Add treat_all_transitions_as_hard! config flag.

Intended for debugging use, primarily. When used, forces all transitions
to happen in :hard mode, so that they raise exceptions.
Also add a :hard mode fail for the invalid_event method.
Includes a unit test to verify the new behavior.
Merge pull request #8 from Originate/add_treat_all_transitions_as_hard

Add treat all transitions as hard