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

utils/pub/command_version.dart

Index: utils/pub/command_version.dart
diff --git a/utils/pub/command_update.dart b/utils/pub/command_version.dart
similarity index 55%
copy from utils/pub/command_update.dart
copy to utils/pub/command_version.dart
index c1fe4f4816b65723c926c8684b30c3770c32da77..a65a5ba84ac257bbc48f1106a4b08577bb97217a 100644
--- a/utils/pub/command_update.dart
+++ b/utils/pub/command_version.dart
@@ -2,7 +2,9 @@
 // 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();
+}