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
This relates to the previous comment for this file, why does it remain available? Does it still serve any purpose other than documentation?