'Register an observer for an event.
Each observer will be registered with a certain priority. Higher
priority observers get called before lower priority observers.
@param event: Name or XPath query for the event to be monitored.
@type event: C{str} or L{xpath.XPathQuery}.
@param observerfn: Function to be called when the specified event
has been triggered. This callable takes
one parameter: the data object that triggered
the event. When specified, the C{*args} and
C{**kwargs} parameters to addObserver are being used
as additional parameters to the registered observer
callable.
@param priority: (Optional) priority of this observer in relation to
other observer that match the same event. Defaults to
C{0}.
@type priority: C{int}'

def addObserver(self, event, observerfn, priority=0, *args, **kwargs):

    self._addObserver(False, event, observerfn, priority, *args, **kwargs)
