Refactor command code and add support for --help and --version.

utils/pub/command_version.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.
 
-/** Handles the `update` pub command. */
-void commandUpdate(PubOptions options, List<String> args) {
-  throw 'not implemented yet';
-}
+/** Handles the `version` pub command. */
+class VersionCommand extends PubCommand {
+  String get description() => 'print Pub version';
+
+  void onRun() => printVersion();
+}