Move userstamp setup into after_initialize hook

Now that the userclass will be set in a config.to_prepare
block in apps we need to run this code last.

lib/alchemy/engine.rb

@@ -36,7 +36,7 @@ class Engine < Rails::Engine
       end
     end
 
-    initializer "alchemy.userstamp" do
+    config.after_initialize do
       if Alchemy.user_class
         ActiveSupport.on_load(:active_record) do
           Alchemy.user_class.model_stamper