Add an `AUTHORS` file and update copyright notices.

Change-Id: Ifb505e5664996c1af41e38376e58ba49864213a3
diff --git a/tools/util.py b/tools/util.py
index 6a71360..f087d86 100644
--- a/tools/util.py
+++ b/tools/util.py
@@ -1,4 +1,4 @@
-# Copyright 2015, ARM Limited
+# Copyright 2015, VIXL authors
 # All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without