Add Content Item Etensions Support and RIN Integration

An external team investigated RIN/ChornoZoom integration. However, before integrating their work, I've created an extensible layer for content items that minimizes dependencies from CZ to RIN or any other third-party. Once this framework got in place, I integrated the partner code with ChronoZoom.

Code Review: http://mrccodereview.cloudapp.net/ui#review:id=1309
Deployment: http://cz-july2013-javierluraschi-fixes.azurewebsites.net/sandbox/extensions/