Refunds and Multiple Currencies

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:
1. It's useful
2. 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.