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.
Sponge uses a different subscription marker for example.  I guess at minimum, there would need to be a way to specify how methods are marked as handler methods.