v2.0: NEW GATEWAY: Square (SquareUp) support added!
v2.0: New API functionality to allow setting a `payment_method_uri` on `cart` resources, to specify a specific previously-used payment method.
v2.0: Braintree error handling improvements for certain types of rare error messages.
v2.0: Stripe Connect fix/improvement to better handle the description parameter for very large carts