CRM-12515: better credit card handling in contribution forms.

css/civicrm.css

i/cc-amex.gif

i/cc-amex.png

i/cc-discover.gif

i/cc-discover.png

i/cc-jcb.png

i/cc-mastercard.gif

i/cc-mastercard.png

i/cc-visa.gif

i/cc-visa.png

templates/CRM/Core/BillingBlock.js