FIX: optimized images url when using a CDN

The change made to `image_optimizer.rb` provides compatibility with Rails when using a CDN defined in the `config.action_controller.asset_host` setting.

I've also updated `cooked_post_processor.rb` so that it takes into account the *eventually* defined CDN setting when trying to do its magic lightbox treatment.

/cc @SamSaffron & @Supermathie