Fixed some flaws in StatefulTrait.

Hi,

I tried to fix two things in the StatefulTrait:
* The first thing is to add the ability to call ->apply('my_transition') instead of ->my_transition() for objects using the StatefulTrait.
* The second thing is to add an exception at the end of __call() because without it, any call to an undefined method fails silently which is *very* painful.

Feel free to tell me what you think of these changes.

Best regards.