Support GoCardless's 'borderless payments'

[artfulrobot]

GC have introduced something they call Borderless Payments which seems to let people sign up in their own currency and then convert it to GBP for you.

It's unclear to me from the link above and their update API docs how this works.

The data GC sends could look like one of the following:

1. "9.40 GBP (we took this as 10USD and used an exchange rate of 1.234 and charged 60p)
2. "10 USD (we will use an exchange rate of 1.2345 and charge you 60p and you'll get about 9.40GBP)"

If it's (1) then this extension should function just fine, but I suspect it's more likely to be (2), and therefore some more work would need to go into developing support for it.