def add(self, msg, callback, override=False):
        """Add a callback to the callback list.

        msg: Message template.
        callback: Callback method
        override: True - replace all existing callbacks for that template
                  False - append the list of callbacks for that message
                  Default is False
        """
        if override:
            if isinstance(callback, list):
                self._dict[msg] = callback
            else:
                self._dict[msg] = [callback]
        else:
            cb = self[msg]
            cb.append(callback)
            self._dict[msg] = cb
Add a callback to the callback list.

        msg: Message template.
        callback: Callback method
        override: True - replace all existing callbacks for that template
                  False - append the list of callbacks for that message
                  Default is False