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