Fix to successful payment status when PENDING

The driver was returning that a transaction was successful when it was PENDING. Although it has not failed, the transaction has also not yet completed. The isSuccessful() method should only return true when the transaction is successfully completed.

A PENDING transaction is one that will get a final status sent through later, so the merchant site needs to be ready to receive it as a back-end notification.

Released on a point release as it could have a knock-on effect on existing logic. It is tracked under issue #9