Add "shippable" checkbox to product feature settings

The attached patch adds a "shippable" checkbox to the product feature settings, similar to uc_file, so you can specify that a node access feature is not shippable. If no shippable products are in the cart, the delivery address and shipping quote will not be requested during checkout.

After patching you will need to run update.php to add the new field to the database.

This patch was sponsored by PlanesTV.