Added priority support to events

This adds priority support to the event system.

A transform is used to add the 4 extra priority levels (PRE, AFTER_PRE, BEFORE_POST and POST) to the forge EventPriority enum.

This means that there is a 1 to 1 conversion from the Order enum used by SpongeAPI and the EventPriority.

The SpongeEventManager is updated to pass the converted priority to the forge event system.  It also sorts the FML events when any new listener is added to the array.