Add a "help" command to Pub.

utils/pub/command_help.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_help');
+
+#import('pub.dart');
+
+/** Handles the `help` pub command. */
+class HelpCommand extends PubCommand {
+  String get description() =>
+    "display help information for Pub";
+
+  String get usage() => 'pub help [command]';
+
+  Future onRun() {
+    if (commandOptions.rest.isEmpty()) {
+      printUsage();
+    } else {
+      pubCommands[commandOptions.rest[0]].printUsage();
+    }
+  }
+}