Add a "help" command to Pub.

utils/pub/command_update.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #library('command_update');
 
 #import('pub.dart');
 
 /** Handles the `update` pub command. */
 // TODO(nweiz): Make update do something different than install when we have
 // sources for which that makes sense.
 class UpdateCommand extends PubCommand {
   String get description() =>
     "update the current package's dependencies to the latest versions";
 
+  String get usage() => 'pub update';
+
   Future onRun() {
     print("Not implemented.");
   }
 }