New version of SHACL specification

As discussed last week, I have updated the SHACL spec

     http://w3c.github.io/data-shapes/shacl/

I applied the various metamodel changes leading to changes in 
terminology. I have also tried to improve the level of preciseness and 
overall structure of the document. For this I have used the draft from 
Dimitris/Peter as inspiration. I did not 100% follow the mathematical 
style proposed there because I find that too dry and hard to understand 
at times with all the variables. I believe I have nevertheless found a 
suitable middle ground that is unambiguous yet readable. If you are 
interested in the relation between the current draft and Dimitris' 
version, see

     http://w3c.github.io/data-shapes/shacl/dimitris-core.html

which has sections marked as green (almost identical), yellow (similar) 
or red (different). The new structure allowed me to auto-generate two 
Appendixes with the syntax rules and the validators.

This was a significant piece of work, spanning over 4 days of my time. I 
hope it takes SHACL closer to CR status. I would appreciate a proper 
read-through by everyone who can spare the time. Unless any major issues 
get reported, we could aim at publishing the final public working draft 
in a week from now.

Thanks
Holger