Enable repeat purchases to be done via the purchase (and authorize) methods

[eileenmcnaughton]

trafficstars

This is more compatible with other processors that accept a token or card reference against the purchase and authorize actions in order to process a new payment using information stored on file from previous interactions

Per https://github.com/thephpleague/omnipay-sagepay/issues/157

[eileenmcnaughton]

It occurs to me that if this is merged it makes sense to gut the repeatPurchase & repeatAuthorize methods & have them extend purchase & authorize with perhaps an extra check to ensure the transaction fields are present