Set mode for EventEmitter behaviour. $emitterMode can be one of:
EventEmitter::EVENTS_FORWARD Allows all events to be forwarded (Default)
EventEmitter::EVENTS_DISCARD Disallows all events from being forwarded
EventEmitter::EVENTS_DISCARD_INCOMING Discards only listeners attached to $this emitter
EventEmitter::EVENTS_DISCARD_OUTCOMING Discards only further emits on forwarder