Improve credit card payment method support

[rvdsteege]

The current integration does not have support for the credit card payment method. Credit card can be used through the general "Pronamic" payment method within for example WooCommerce, but not with the "Credit card" payment method.

The reason this support has not been added, has to do with EMS requiring a card brand when starting a new payment. There is no general 'credit card' payment method at EMS, which could result in a hosted payment page to select the card brand (or a uniform card details screen).

We could maybe do something with credit card issuers to select the card issuer at checkout, but with #1 and pronamic/wp-pronamic-pay#154 in mind that would soon be obsolete.