[paymentrequest] Second set of API refactoring changes (#37)

This pull request includes refactoring of the API to add support for payment items, add detail to shipping option support, add detail to event firing including factoring out payment request options from the transaction details, and adding a skeleton algorithm section.

This is the second set of refactoring changes to incorporate feedback we have received.
You can view, comment on, or merge this pull request online at:

  https://github.com/WICG/paymentrequest/pull/37

-- Commit Summary --

  * Clarified JSON object
  * Refactor spec and outline events and algorithm sections.
  * Added warning note to explainer document.
  * Added reference to ISO 4217.
  * Add support for payment items.
  * Add fields for ShippingOption.
  * Refactor PaymentDetails.

-- File Changes --

    M explainer.md (5)
    M specs/paymentrequest.html (518)

-- Patch Links --

https://github.com/WICG/paymentrequest/pull/37.patch
https://github.com/WICG/paymentrequest/pull/37.diff

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https://github.com/WICG/paymentrequest/pull/37