Refund support

Patch to support refunds.

@mmaranao - I've set up an issue fork for this patch. I've not testing it out yet though.

I'm unsure of this line as we have an configurable order number strategy in place. Maybe you can shed some light on the change?

'remote_id' => $payment_receipt['transaction_no'] . '|' . $payment_receipt['order_no'] . '|' . $payment_receipt['reference_no'],

Thanks!