Validate Gateway.supported_countries

Validate that the list of supported country codes sent to
Gateway#supported_countries contains only valid entries (according
to active_utils' ActiveMerchant::Country definition).

This necessitated the modification of some existing gateways'
supported_countries lists. In most cases a mapping to the correct
country code was made. However, in rare cases the invalid country
code was removed and no meaningful new value was substituted.

Closes #1142.