cmd/*: add license headers

cmd/ethereum/main.go

@@ -1,3 +1,20 @@
+// Copyright (c) 2013-2014, Jeffrey Wilcke. All rights reserved.
+//
+// This library is free software; you can redistribute it and/or
+// modify it under the terms of the GNU General Public
+// License as published by the Free Software Foundation; either
+// version 2.1 of the License, or (at your option) any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+// General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this library; if not, write to the Free Software
+// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+// MA 02110-1301  USA
+
 package main
 
 import (