Sets naming convention for relationships, adds docs

This promotes using the names "subject" and "target" for `guid_one` and `guid_two`, respectively. I think the archer/(literal) target metaphor in the docs makes this clearer.

For now I left variables named $guid_one, etc. because I wanted to keep the API consistent with existing ElggRelationship property names.

What does everyone think of deprecating the `guid_one`-type properties using __get/set? It seems like few devs are actually using these.