[Issue #527] Revive common ActionResults - Part 1.

1. Added HttpNotFound() and HttpNotFound(string statusDescription) to Controller.
2. Updated HttpStatusCodeResult to include the status description and expose the
   StatusCode and StatusDescription as properties.
3. Added unit tests for HttpNotFound() and for HttpStatusCodeResult.
4. Updated the MvcSample to add an action that uses HttpNotFound().
just fyi...webapi & mvc currently has NotFoundResult specifically for not found cases...not sure if we want to do similar stuff here...