document graph traversal reimplementation for developers

Documents the changes to server code brought by https://github.com/openmicroscopy/openmicroscopy/pull/3228; some of the links may remain broken until it is merged. Mostly staged at http://www.openmicroscopy.org/site/support/omero5.1-staging/developers/Server/ObjectGraphs.html.
--no-rebase
If we consider `Model/` to be things that client developers will also need to know, then I agree that this belongs under `Server/`.  As for the name in general, I don't know what would be catchiest. "Graph operations"? "Working with graphs of objects"? What's here means something to me, but that's not the most indicative of what it *should* be. :smile: