[paymentrequest] Minor spec changes based on implementation experience and feedback (#44)

Most significant change is to use strings for amount value based on more discussion. Also moved some validation steps in show() earlier.
You can view, comment on, or merge this pull request online at:

  https://github.com/WICG/paymentrequest/pull/44

-- Commit Summary --

  * You must have at least one item (not more than one)
  * Move some validation steps in show() before promise is created
  * Update amount to use a string for value.
  * Update example variable names based on feedback to #37

-- File Changes --

    M specs/paymentrequest.html (61)

-- Patch Links --

https://github.com/WICG/paymentrequest/pull/44.patch
https://github.com/WICG/paymentrequest/pull/44.diff

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https://github.com/WICG/paymentrequest/pull/44