'Add callback.
The arguments passed are used as callback specific arguments.
@param onetime: If C{True}, this callback is called at most once.
@type onetime: C{bool}
@param method: The callback callable to be added.
@param args: Positional arguments to the callable.
@type args: C{list}
@param kwargs: Keyword arguments to the callable.
@type kwargs: C{dict}'

def addCallback(self, onetime, method, *args, **kwargs):

    if (not (method in self.callbacks)):
        self.callbacks[method] = (_MethodWrapper(method, *args, **kwargs), onetime)
