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 
 
I'm of two minds about mirroring Apple's structure so closely. On the plus side:

* immediately obvious mapping

On the negative side:

* means things get confusing if Apple renames things (which is likely since this is so new)
* obscures the actual protocol
* makes the solution very ApplePay specific, whereas Apple Pay is really an implementation of a developing industry standard (EMV)

You can probably tell I lean towards being less Apple specific. I'd actually like:

```ruby
class ActiveMerchant::Billing::Cryptogram < Model
  attr_reader :encrypted_data, :description, :brand

  def initialize(encrypted_data, options={})
    @encrypted_data = encrypted_data
    @description = options[:description]
    @brand = options[:brand]
  end
end
```

Note that `transaction_identifier` isn't there; that's because I think it should be provided to purchase, as with other payment methods:

```ruby
gateway.purchase(amount, cryptogram, order_id: transaction_identifier)
```

I think this API will have much wider applicability, exposes the underlying protocol, future proofs us from Apple API shenanigans, and give us a place to hang deeper Cryptogram support, since I hope we'll be able to get at the innards eventually.

If there's a concern that devs will not know how to use it, we could provide a helper method:

```ruby
cryptogram = Cryptogram.from_apple_pay(payment_data, payment_instrument_name, payment_network)
```

Thoughts?