[w3c/browser-payment-api] Payment app discovery (#155)

The browser payment api spec suggests that the user-agent will display the intersection of payment apps that the merchant website supports and the user has installed. Many payment apps that Shopify supports do not require user registration as they either 1) provide an onboarding process as part of payment or 2) provide the option of processing credit cards directly.

PayPal is a good example of this. Once a merchant enables PayPal on their Shopify store, it is always presented as an option in checkout. If a buyer does not have a PayPal account, they are given the option to create one or pay via credit card. In the context of the `PaymentRequest`, a merchant would include  PayPal as a `supportedMethod`. The buyer may not have registered PayPal as a supported PaymentApp, however we do not want to exclude it from being a valid payment method to use.

<img width="350" alt="paypal checkout - create a paypal account 2016-04-18 15-02-17 2016-04-18 15-02-24" src="https://cloud.githubusercontent.com/assets/2962326/14745418/42423078-0879-11e6-8e81-1a6ac5cdec07.png">

There are also payment apps that may be particularly useful for the specific goods the user is selling. For example bitcoin or net banking for virtual purchases. Or, financing methods for larger purchases.

Below is a screenshot of what the last step of our checkout looks like with multiple payment options installed by the merchant. 

<img width="550" alt="paypal checkout - create a paypal account 2016-04-18 15-02-17 2016-04-18 15-02-24" src="https://cloud.githubusercontent.com/assets/2962326/14745443/5a10b8e6-0879-11e6-9317-f0ad353e0d51.png">

Should the ability for a merchant to categorize payments apps as `alwaysShow` or `suggestedApps` be defined in the spec? `suggestedApps` would be those apps that the merchant would always want the user agent to provide as option to the user whether they have them installed / registered or not, this would also be a way for users to discover new payment apps.

**Note:** There is some overlap between payment method discovery which has been [discussed previously](https://github.com/w3c/webpayments/issues/110) and payment app discovery but I think it's worth defining both issues seperately. 

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