Add _links_templates for generating URL templates

If a serializer responds to this method, its value is then
used as value for "links" in root of the document.
When runnin w/in Rails, make available url_helpers

This technique ties this usage to Rails, which but makes the helpers
directly accessible w/in any instance of ActiveModel::Serializer,
including any associations.

Conflicts:
	lib/action_controller/serialization.rb
	lib/active_model/array_serializer.rb
	lib/active_model/serializer.rb
	test/fixtures/poro.rb
Fixed loading url helpers