Add URL to serializers (closes #61)

Thanks so much to @vanstee for the initial
implementation!

test/test_helper.rb

@@ -20,11 +20,13 @@ require 'rails'
 module TestHelper
   Routes = ActionDispatch::Routing::RouteSet.new
   Routes.draw do
+    resource :hypermedia
     match ':controller(/:action(/:id))'
     match ':controller(/:action)'
   end
 
   ActionController::Base.send :include, Routes.url_helpers
+  ActiveModel::Serializer.send :include, Routes.url_helpers
 end
 
 ActiveSupport::TestCase.class_eval do