The array keys are event names and the value can be:
* The method name to call (priority defaults to 0)
* An array composed of the method name to call and the priority
* An array of arrays composed of the method names to call and respective
priorities, or 0 if unset
For instance:
* array('eventName' => 'methodName')
* array('eventName' => array('methodName', $priority))
* array('eventName' => array(array('methodName1', $priority), array('methodName2'))