[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().
Need to have a discussion about the utility of bringing back the derived versions of this. http://msdn.microsoft.com/en-us/library/system.web.mvc.httpnotfoundresult(v=vs.118).aspx.

I don't feel strongly about bringing it back (personal gut feeling) but it needs to be considered. @Eilon I summon you!