Tweaked factory_girls_initialization process to allow other engines to advertise its factories for use in a recipient project

When a (mountable) engine defines models and factories to test the models, sometimes it is convenient to use this factories
from a project which depends on the engine.

But current factory_girl_rail initialization processes do not allow to do so without hacks.
(after_initializer resets all FactoryGirl.definition_file_paths to defaults with = [paths])

When this change introduces the engine can advertise some factories in an its own railtie, i.e.:

module ModelCore
  class Engine < Rails::Engine

    initializer "model_core.factories", :after => "factory_girl.set_factory_paths" do
      FactoryGirl.definition_file_paths << File.expand_path('../../../spec/factories', __FILE__) if defined?(FactoryGirl)
    end
  end
end

lib/factory_girl_rails/railtie.rb

@@ -14,12 +14,15 @@ module FactoryGirl
       end
     end
 
-    config.after_initialize do
+    initializer "factory_girl.set_factory_paths" do
       FactoryGirl.definition_file_paths = [
           File.join(Rails.root, 'factories'),
           File.join(Rails.root, 'test', 'factories'),
           File.join(Rails.root, 'spec', 'factories')
       ]
+    end
+
+    config.after_initialize do
       FactoryGirl.find_definitions
     end
   end