[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().
@rynowak The documentation is copied from the one on the MSDN http://msdn.microsoft.com/en-us/library/system.web.mvc.httpstatuscoderesult(v=vs.118).aspx

Although I accept if you think it suck and I'll change it.