Add default justify classes.

on( eventName, listenerFunction, [scopeObj], [listenerData], [priority] ) : Object

According to ckeditor documentation each command have it's own prority. If listener cancels event - other listener won't get notification. So only listener with lowest priority will be called. I lowered each align command "exec" listener priority to -1.

Added ability using justifyClasses setting to add class to aligned image.