Allow storage of a default Billing Address on B2C sites.

We have some B2C sites that take both credit cards and allow for future internal billing. Since these sites do not utilize locations, the billing address at checkout defaults to their default address book entry. It would be ideal if they could define both a default shipping address as well as a default billing address. This would be especially beneficial for those whose billing address for a credit card payment does not match the destination for shipping.