Refactored with helper method to move the ghost.
fixed typo in comment.
No need for PacmanInteraction to extend Observable.
Typo in message to user fixed.
Ensured state machine consistency upon game modification.

Added explicit invariant to PacmanInteraction, which ensures
consistency between the underlying game the state machine.
Game informs via update if it has changed, and the updates 
restores consistency if necessary.
Removed obsolete unused method.
Merge pull request #5 from avandeursen/statemachine

Fixed state machine consistency.