Add @graingert as a CONTRIBUTOR

CONTRIBUTORS.txt

@@ -121,3 +121,5 @@ Thomas Meson, 2012/08/28
 Daniel Lundin, 2012/08/30
 Alexey Zatelepin, 2012/09/18
 Sundar Raman, 2012/09/24
+Henri Colas, 2012/11/16
+Thomas Grainger, 2012/11/29