ISIS-917: first cut at a pluggable representations service for RO.

The app runs fine, but tests are failing.
ISIS-917: first cut at a pluggable representations service for RO.

Fix for NPE when app runs (previous commit lied), tests still failing.
ISIS-917: fixing TCK tests for RO viewer post-refactoring.
ISIS-917: further refactorings to simplify the API of the (newly rechristened) RepresentationService.