[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 Look at the bug #527 and you'll see that I comment on what HttpNotFoundResult does in MVC and that there is no equivalent in the current world. (And I'm not sure if it should be one).

HttpNotFound does something different in MVC than what it does in Web API. I think it makes sense to be content negotiated if you are returning an object (for which there are other alternatives that generate the same result) but not if you are just returning a string in plain text (which is something I would say all http clients can understand without problem).