New proposal for RDF* Semantics

Hi all,

I was hoping to send this earlier so that we could discuss this during 
our next call, but given the short delay, it will have to wait for a 
later call.

However, I just pushed a PR which contains a new version of the "RDF* 
Semantics" section This is the result of lengthy discussions with Olaf 
and Doerthe (huge thanks to them), as well as discussions on the mailing 
list and valuable feedback from Peter and Antoine in particular.

     https://github.com/w3c/rdf-star/pull/81

It follows the idea of making RDF* syntactic sugar on top of RDF (cf 
issue 37), at least at the abstract syntax level. Rather than 
reinventing a semantics from the ground up, RDF* semantics is now 
defined as a semantic extension (a.k.a. entailment regime) of RDF 
(similarly to RDFS or OWL).

Yet, it aims to avoid the pitfalls of a full-fledged syntactic sugar 
approach. More precisely: it tries to avoid users from describing 
ill-formed or incomplete RDF* triples using plain-RDF syntaxes.

Any feedback welcome.