Standardise card token/pseudo pan/cardReference

[judgej]

Some discussion for OmniPay 3.0 here:

thephpleague/omnipay#387

If a standard/convention for card tokens is developed for 3.0, then it would be great to backport it here. If we standardize on naming the token cardReference for example, then methods such as getToken() would need to be changed or aliased.

Similarly, passing the token into messages such as ShopServerAuthorizeRequest should use the agreed method and maybe not piggy-backing on the creditCard object.