Recording payment method used

[maiksprenger]

We recently chatted about an Oscar-friendly way to expose the payment method that a user selected on Adyen. It's not pressing for us to implement, so I'm recording our findings in this issue.

Looking at payment.Source and payment.SourceType, it looks entirely reasonable to me that django-oscar-adyen is allowed to get_or_create an Adyen source, and create a SourceType instance for each payment when recording successful payments.

As we already record the Adyen transaction ID in PaymentEvent, I would feel comfortable
recording the payment method in payment.Source.reference. Sounds sane, and it's not unlikely that was the intention of the (rather vaguely defined) field.