DirectMessage added to default plugins

I think the migration from core to plugin is done now for DMs.

This is required since we support the Twitter-based API by default,
which is implemented in many of the mobile clients etc. But you can
disable the DirectMessage for your instance of you wish, of course.

lib/default.php

@@ -303,6 +303,7 @@ $default =
                             'Bookmark' => array(),
                             'ClientSideShorten' => array(),
                             'Directory' => array(),
+                            'DirectMessage' => array(),
                             'EmailAuthentication' => array(),
                             'Event' => array(),
                             'Oembed' => array(),