Add class name to uc_credit card icons

The attached patch adds a "uc-credit-cctype" class and a "uc-credit-cctype-XXX" class (e.g. uc-credit-cctype-visa) to each of the credit card type icons on the checkout form. This allows for additional theming and js functionality.

My use case was that based on the currency selected (with uc_multiprice module) the supported card types had to change. I was able to change the drop-down with a hook_form_alter() but to change the icons displayed I need to be able to target the individual img tags so I could hide/show them using custom javascript. Adding class names to the img tags allowed that.