Include page wrapper CSS and JS files last

This patch adds support for CSS weight in the Page Wrappers module. This adds a 'weight' parameter to the $options array when running drupal_add_css() and drupal_add_js(). This should make sure the CSS and JS files are loaded in the proper order—specifically, to make sure custom uploaded CSS loads later than system and theme CSS.

This should be a quick and easy merge. We considered adding fields to the page wrappers config to set the weight explicitly, but it seemed that this would be overkill for 99% of use cases. Happy to make further adjustments in the future.