Link "excluding shipping costs" to a details page

Information for shipping details and costs must be easily available in on-line shops in Germany, so it is best to add a link from the "excluding shipping costs" to a separate page with detailed information about shipping. The attached patch introduces a textfield in the VAT admin form that can contain an optional path to another node or page. If this field is non-empty "shipping costs" will be linked to that page.

This feature was already requested here http://drupal.org/node/490646#comment-1732870