SagePay: Add support for GiftAidPayment

This is an optional flag to indicate that the payment is a Gift Aid
charitable donation and the customer has agreed to donate the tax.

From the SagePay docs:

![Screenshot_1_31_13_2_53_PM](https://f.cloud.github.com/assets/2437/115948/7ba69f74-6be3-11e2-9ff9-cd76a011de65.jpg)