Add a "help" command to Pub.

utils/pub/command_install.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_install');
 
 #import('entrypoint.dart');
 #import('pub.dart');
 
 /** Handles the `install` pub command. */
 class InstallCommand extends PubCommand {
   String get description() => "install the current package's dependencies";
 
+  String get usage() => 'pub install';
+
   Future onRun() {
     return entrypoint.installDependencies().transform((_) {
       print('Dependencies installed!');
     });
   }
 }