Changes from extracting bits to ActiveModel
 
* ActiveModel::Observer#add_observer!
 
  It has a custom hook to define after_find that should really be in a
  ActiveRecord::Observer subclass:
 
    def add_observer!(klass)
      klass.add_observer(self)
      klass.class_eval 'def after_find() end' unless 
        klass.respond_to?(:after_find)
    end