Charge customers in their local currency

[Lanny]

Currently we charge in pounds, for some customers this may incur a currency exchange fee if their card operates in a different currency than pounds. Stripe lets us transact in currencies other than GBP so the parts of this are:

1. Determine a currency for the user. Maybe mapping some locale/language header? Or using IP addrs?
2. Figure out how to present prices in the user's currency (presumably stripe gives us some utility for doing this as rates fluctuate)
3. Charge in the target currency