Move AR include into engine file.

lib/impressionist/engine.rb

@@ -2,11 +2,11 @@ require "impressionist"
 require "rails"
 
 module Impressionist
-  class Engine < Rails::Engine    
+  class Engine < Rails::Engine
     initializer 'impressionist.extend_ar' do |app|
-      ActiveRecord::Base.extend Impressionist::Impressionable
+      ActiveRecord::Base.send(:include, Impressionist::Impressionable)
     end
-    
+
     initializer 'impressionist.controller' do
       ActiveSupport.on_load(:action_controller) do
         include ImpressionistController::InstanceMethods