Bind an event to a call function and ensure that it is called for the
        specified event
        :param event: the event that should trigger the callback
        :type event: str
        :param callback: the function that should be called
        :rtype callback: function

def bind (self, event, callback):
        """
        Bind an event to a call function and ensure that it is called for the
        specified event
        :param event: the event that should trigger the callback
        :type event: str
        :param callback: the function that should be called
        :rtype callback: function
        """
        if self._events.has_key(event):
            self._events[event].append(callback)
        else:
            self._events[event] = [callback]