Fix typo retrieving vendor prefix

Properly get the OSCAR_SAGEPAY_TX_CODE_PREFIX from the project settings
Compatibility patch for Oscar 0.4

These were added or changed after Oscar 0.4:
- oscar.apps.payment.bankcards was added : copy it over
- address AbstractCountry's code was added : fallback to iso_3166_1_a2
- Bankcard has changed some fields : fallback to the od names