Add copyright header to copied Gogs code

stats.go

@@ -1,3 +1,7 @@
+// Copyright 2014-2018 The Gogs Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style license that can be
+// found in the LICENSE file of the Gogs project (github.com/gogs/gogs).
+
 package writefreely
 
 import (