Multiple currency support added to the existing doDirectPayment method because:
It's useful
PayPal returns a misleading "You can not refund this type of transaction (10009)" message if you are using a Sandbox account that is not setup with the currency of the original transaction when attempting a refund.
RefundTransaction support added to allow for refunds to be processed to a PayPal users account for transactions less than 60 days old. (https://developer.paypal.com/docs/classic/api/merchant/RefundTransaction_API_Operation_NVP/)
Multiple currency support added to the existing doDirectPayment method because: