PR#109: Editorial polishing of abstract, intro material, terminology, etc.

Howdy folks,

fyi, I took a stab at editorial polishing of abstract, intro material,  
terminology, etc.  The PR is here..

   https://github.com/w3c/webauthn/pull/109

It explicitly includes a bunch of Vijay's recent commits (which have  
already been merged to master) because of the way I created my branch,  
my commits are down at the bottom of the commit list.

Also, there is a textual diff in the  
"vijaybh-lots-of-fpwd-issues-jeffh-build" branch here..

    
https://github.com/w3c/webauthn/blob/vijaybh-lots-of-fpwd-issues-jeffh-build/diffs/diff-webauthn-vijaybh-lots-of-fpwd-issues-jeffh-build-e799e94-2016-05-20--from--webauthn-master-2016-05-20-0950a1d-index.pdf

..github will render it in the browser (cool!), although it doesn't  
seem to work every time -- it's also attached to this message.

the PR detailed description is included below.

HTH,

=JeffH

https://github.com/w3c/webauthn/pull/109
The commits represented in this PR comprise the following edits:

     Polish abstract.

     Convert "Use Cases" section into an "Introduction" containing a  
"use cases" subsection.

     Add explanatory material to Introduction.

     Make modest clarifying alterations to the use cases proper.

     Polish/clarify "Conformance" section.

     Add RFC2119 statement and reference.

     Move "Terminology" section to be after "Conformance".

     Polish/Expand "Terminology" section.

     Move biblio.json into index.src.html and rename biblio.json to  
biblio.json.bak

     Added some folks to #acknowledgements.

Note 1: this PR includes an experiment regarding section titles that  
refer to WebAuthn API WebIDL objects -- altered the WebAuthn Assertion  
section to contain a link directly into the IDL rather than be  
self-referential -- seems more useful to me, WDYT ?

Note 2: this PR creates a diffs subdirectory, containing a diff of  
rendered plain-text of this index.src.html from the latest in master.  
See  
https://github.com/w3c/webauthn/tree/vijaybh-lots-of-fpwd-issues-jeffh-build/diffs

Fixes #92.

makes some progress on #66 et al.