Attempt to make turtle represent Agent and get a good diagram.

This PR is for feedback at this stage. The picture is getting to where I more or less like it. I don't know if the Turtle is correct, @pmackay

Note: I created a subClassOf rdf:Property called RelationshipType. elf had suggested this in a gitter conversation, but we certainly didn't decide it, and elf suggests a lot of things just to brainstorm and get us to think. :) It does however give us a way to keep all the relationships gathered in one place kind of. Otherwise all properties will eventually go there. @ahdinosaur what do you think of this (and everything else).

Also, still using vf:label. And can't remember or find a standard vocab for urls that isn't @id as in json-ld. This is for the url and the image on Agent. I'll figure it out, just noting it isn't right.

Since I don't want to merge it, here is what it looks like in vowl: