Basic support for obtaining UPS shipping labels (Rebased)

"I've added very basic support for obtaining UPS shipping labels. This also provides support for Saturday delivery, negotiated rates, and different pickup/return addresses. Let me know what you think, I'd love to get these changes merged." - @jmorton
Let's call this ```create_shipment```, to keep in line with how we do it here for Canada Post. That should then be the standard for all carriers.