[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().
@kichalla And there are two anwers for that:
1. It can be handled by another overload that takes an error object and does content negotiation over it.
2. If you use a client, let's say jQuery and I send you back a 404 with text/plain and an error you are still going to be able to read it and interpret it correctly, while the opposite is not always true.