Refactor use of event render JS hook to allow greater flexibility

[stephenharris]

Currently, if an event fails any of these conditions then it will not display. The JS hook eventorganiser.fullcalendar_render_event can only be used to remove an event from the calendar - it should be allowed to stop the hiding of an event, based on some other logic.

This issue was prompted by this request: http://wp-event-organiser.com/forums/topic/hierarchial-event-venues/. Specifically refactoring the use of the hook so it is triggered before returning would allow the user to over-ride the logic of excluding the event based on the venue filter.