Added extra priorities to support Sponge

Sponge has 4 extra priority levels.  This expands the EventPriority enum so that the extra priorities are supported.

It also adds a .getBusID() method to the EventBus class.  This means that the Sponge event system can directly submit events to the ListenerLists associated with Forge events.
It means we can do a one to one mapping of the priorities.