Support refunds

[judgej]

Not pressing or urgent, but the gateway supports it, so would be nice to complete the breadth of support.

[judgej]

Other Server methods that this gateway provides include, but are normally out of scope for OmniPay:

• 3dscheck (3D Secure verification on its own (without being wrapped into other flows).
• bankaccountcheck
• addresscheck
• consumerscore

There does not appear to be a way to explicitly fetch a past transaction, unless I'm just not seeing it.