[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().
@Eilon I'll bring it back then in a separate PR. @yishaigalatzer are you ok with that? (You had comments on the issue regarding this topic).