[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().
regarding "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)."...why are we making these assumptions?...an ajax client communicating in json format and users sending back additional error text with a `not found` response is a common scenario...