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.
Would you support direct listener registration (and unregistration), rather than passing the object?

    
    private void register(Class<?> eventType, IEventListener listener, EventPriority priority, ModContainer owner)