[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().
Changes from MVC5 to MVC6 need to be very explicit and well thought out. If it existed before, the default is that it should continue to exist. We've been seeing this pain already show up during migrations.