added Łukasz Langa to contributors list

CONTRIBUTORS.txt

@@ -111,3 +111,4 @@ Matt Long, 2012/07/04
 David Markey, 2012/07/05
 Jared Biel, 2012/07/05
 Jed Smith, 2012/07/08
+Łukasz Langa, 2012/07/10