Add Adrien Guinet as contributor

CONTRIBUTORS.txt

@@ -139,3 +139,4 @@ Dong Weiming, 2013/06/27
 Oleg Anashkin, 2013/06/27
 Ross Lawley, 2013/07/05
 Alain Masiero, 2013/08/07
+Adrien Guinet, 2013/08/14