cmd/geth: added copyright and license information

As per GNU GPL requirement I've added the copyright and the license
information as a subcommand as well as a copyright notice when
displaying the help command.

cmd/geth/usage.go

@@ -30,6 +30,8 @@ import (
 var AppHelpTemplate = `NAME:
    {{.App.Name}} - {{.App.Usage}}
 
+   Copyright 2013-2016 The go-ethereum Authors
+
 USAGE:
    {{.App.HelpName}} [options]{{if .App.Commands}} command [command options]{{end}} {{if .App.ArgsUsage}}{{.App.ArgsUsage}}{{else}}[arguments...]{{end}}
    {{if .App.Version}}