added silent relationships

Placeholder for review, needs more tests and better comments and depends on #2260 being merged
In Orbit, forward and reverse links are tracked as `__rel` and `__rev`, which could replace `_relationships` and `_silentRelationships` here. Not sure if you want to use the same terms, but I did arrive at them based on [standard HTML `link` attributes](http://www.w3.org/TR/html401/struct/links.html#h-12.3.1). Also, for the sake of simplicity and maintenance, Orbit tracks all reverse links per model in one hash.