Edits based on recent discussions and decisions

[ianbjacobs]

This was not a comprehensive review of the document. However, I
reviewed it in light of some recent decisions and did some updates:

• Given 4 January discussion about recommended payment apps, I reviewed
  the text on that topic and made changes.
• I also cleaned up some text on registration given the new assumption
  that registration IS a prerequisite for usage of a payment app that
  conforms to this specification.
• It has also become clear that this specification is not designed to
  address native payment apps, and so I deleted some text on native
  payment apps, and also stated more clearly that those mechanisms lie
  outside the scope of this document.
• IMPORTANT: It is my current understanding that a payment app
  identifier will designate service worker code. Therefore, the spec now
  says that; we’ll discuss whether this was the right edit at our 10
  January call.
• I tried to reduce instances of the word “display” in light of recent
  discussions about “enabling the user” rather than always talking bout
  display of information. Nonetheless, some instances of “display” still
  remain in the spec where they make sense.
• I added mention of HTTP Link headers for finding payment app manifest
  files; we may or may not need that but I’m keeping this spec aligned
  with the payment method stuff.
• I added mention of paymentRequestID and otherwise cleaned up
  discussion of reconciliation.
• Now that we have canHandle I removed some other notes and provided a forward reference to it.
• We also need to figure out (cf issue 79) how and when the payee provides information about recommended payment apps.

[ianbjacobs]

I am merging this PR after brief email with AHB. There are differing views on the PAI, so I've added a pointer to issue 48 and we'll use the revised text as we continue the discussion.