Support for payments to the U.S.

In order to transfer money to the U.S. (and other countries), there need to be two more hacks.

* Set *Zahlungsartschlüssel* (T.22) to `00` (= general purpose SWIFT transfer) instead of `13` (*EU-Standardüberweisung*)
* Use the bank account number instead of the IBAN.

There is one more bug:
Bank account numbers in other countries may be longer than 10 chars, and bank codes may be longer than 8 chars. As the DTAZV part uses `Account`, it breaks when they are. Maybe I'll fix this later.

No tests included, but works in production.