Enables priority on events.

(from https://github.com/stonetwig/fullcalendar) Example: event.priority = 10 will get hig… …
…ht up than event.priority = 5, documentation: http://stackoverflow.com/questions/15226946/render-event-on-top-fullcalendar. No test available since it's such a little change, won't effect those who doesn't want to prioritize by anything special..