New doc: Options for RDF Expression of Credibility Data

I made a ton of changes, trying to take everyone's comments into 
account. I gave it a different title and URL, so as not to step all over 
the comments on yesterday's draft.

New version is Options for RDF Expression of Credibility Data 
<https://docs.google.com/document/d/1-Z5gsTiqlJf7SGdyJ0N8ZesgOVurwGB_sdrmMAcnZ2g/edit>.

I'm afraid the document may look a bit hostile toward shape 3 (named 
graphs), because several people have now said they prefer that approach, 
so I needed to get into some of the not-so-obvious downsides. I'm pretty 
sure I don't actually have a bias here, and I'm personally quite open to 
be convinced by issues I haven't considered. The actual code changes in 
switching between shapes 1, 2, and 4 are trivial; shape 3 requires some 
issues be solved first, as now discussed in the draft.

I also added JSON-LD versions, to go along with SPARQL and images.

      -- Sandro