Re: [webpayments] How do organizations layer additional information in the core messages? (#18)

>From [Payment Request Architecture](http://wicg.github.io/paymentrequest/specs/architecture.html#payment-transaction-message-specifications):
> If we choose to support well-known short strings for payment methods then we will need to determine where to define them. We may choose to define these well-known strings in a formal specification.

You can have well-known short strings for payment methods via a JSON-LD Context (another reason to use JSON-LD). For example, this is easy to do:

```jsonld
{
  "@context": "https://w3c.org/vocabs/payments/v1",
  "type": "PaymentInstrument",
  "method": "Visa",
  "paymentService": "https://psp.example.com/pay"
}
```

which formally means (via a JSON-LD Context):

```jsonld
  "https://w3c.org/vocabs/payments#method": "https://visa.com/vocabs#Visa"
  "https://w3c.org/vocabs/payments#paymentService": "https://psp.example.com/pay"
```

No danger of conflicts, formal meaning for what "Visa" means, and the benefit that the browser nor developers need to care about those details (they use the markup at the top, which looks more or less like JSON, which they're used to).

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