add credit card interface for mapping to omnipay format

Just a simple interface class that credit card entity's can implement so they have a method for mapping to Omnipay format before submitting to a gateway. 

I considered putting constants for all of the Omnipay properties, or writing a "mapper" class that implemented this interface and had a check for Omnipay methods named "set[whateverProperty]" but those both seemed too heavy.

This will just act as a simple reminder to implement a mapper to Omnipay format and supply the convenience method for doing that mapping.