[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().
I don't think HttpNotFoundResult adds a lot of value to the table, and having a method on Controller for returning that type of result makes it less likely to break people as I don't expect people to be newing up instances of HttpNotFoundResult directly when they can just call the method on controller.

Other possible scenarios where this might be used are ActionFilters but I think that's a much more advanced scenario, so in conclusion, I would say don't bring it back unless we have evidence it's being used.