Unbind the callback from the event and ensure that it is never called
        :param event: the event that should be unbound
        :type event: str
        :param callback: the function that should be unbound
        :rtype callback: function

def unbind (self, event, callback):
        """
        Unbind the callback from the event and ensure that it is never called
        :param event: the event that should be unbound
        :type event: str
        :param callback: the function that should be unbound
        :rtype callback: function
        """
        if self._events.has_key(event) and len(self._events[event]) > 0:
            for _callback in self._events[event]:
                if _callback == callback:
                    self._events[event].remove(callback)
                if len(self._events[event]) == 0:
                    del self._events[event]