Add Adyen direct gateway.
Rename message and success.
Reduce the reliance on instance variables.
Purchase using MultiResponse.
Make merchant a credential.
Fix credentials in unit tests.
Remove extra merchant options in Adyen tests.
Add _hash prefix to adyen specific methods and support options[:address].
Rename parse_reponse to parse.
Simplify response parsing.
Parse addresses for Adyen in a simple way.
Better ResponseError management.
Better 500 errors management.
Implement feedback fixes.
Error fallback.
Fail on 404.
Simpler success_from.
Add countries from signup form.
Bogus gateway requires CVV
Wirecard: Use authorization_check for Amex store

Amex does not support authorization_check, so an authorization is done
instead.

Closes #1258.