Issue 2 proposal v13

Attached.

Included Dave's inlined changes, addressed his comments (except the ones 
related to URI values -- see my response to his email), fixed 
indentation of examples, fixed one UUID for request messages (in the 
examples) to make them all unique, addressed all of SimonN's comments.

Looking at v13, there are a few things that are worded in an awkward 
way, since the origin of this proposal lies in trying to get a 
directional proposal so that the TC could agree on the protocol 
elements/how it works. But we have moved on to agreeing on exact 
wordings. I think this is purely editorial and can be fixed later (as PR 
comments) so won't bother with making/suggesting any changes now.

With this, I believe all the outstanding comments except one (see below) 
are addressed. I'm hoping we can resolve issue 2 with this.

The only outstanding issue is whether to change 'SCA reference' to 
something like 'the invoker' or 'the client' so that non-SCA clients can 
use it.

-Anish
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