2Checkout new features

* Added currency code param so that merchants can price in multiple currencies.

* Changed purchase routine URL from suffix from 'spurchase' to 'purchase' as 2Checkout no longer has a separate single page checkout.

* Changed the callback function to look at all returned params so that Header Redirect return method (returns by GET) can also be used.

* Added support for 2Checkout's new Direct Checkout integration.

Thanks, and let me know if you have any questions or concerns.