_build_event: Ensure __name__ is a str

Python 3 is happy to receive `__name__` objects as Unicode or plain
strings, but Python 2 is not.  This bug shows itself when
`unicode_literals` is imported from calling code.

mriehl#31

src/main/python/fysom/__init__.py

@@ -302,7 +302,7 @@ def _tran():
                 self._reenter_state(e)
                 self._after_event(e)
 
-        fn.__name__ = event
+        fn.__name__ = str(event)
         fn.__doc__ = ("Event handler for an {event} event. This event can be " +
                       "fired if the machine is in {states} states.".format(
                           event=event, states=self._map[event].keys()))