add ApplePayPaymentToken model

So, Apple Pay is officially out in the wild. In lieu of credit cards or e-checks, [Apple Pay Payment Tokens](https://developer.apple.com/library/IOs//documentation/PassKit/Reference/PKPaymentToken_Ref/index.html) will be flying around. 

Following the **Billing::CreditCard** and **Billing::Check** model pattern, this adds **Billing::ApplePayPaymentToken** motivated by the API doc above. 

@edward @girasquid /cc @joshpc