Port uc_save_for_later to D7

I'm attaching a stripped down D7 port of the Save for Later module, which I've tested with Drupal 7.0 and Ubercart module v3.x, for review. I am not able to maintain this port alone (since I didn't author the D6 module).

The large majority of the work was simply to port code from D6 to D7, and also make changes to port from UC v2.0 to UC v3.x.

Please note the "stripped down" bit is that I removed the ability to save individual shopping cart items during the port, since that wasn't needed for my current application. You can only save all items in the cart (which empties the cart), or retrieve all saved items, adding them to existing cart, although restoring the original feature set to the D7 port is not impossible.