Conflict with other modules

guestbook_user() needs a small tweak to allow better theming and prevent conflict with other modules that are similarly not properly coded (like audio module).

Change this:

$items[] = array(
            'title' => t('Guestbook'),
            'value' => $link,
            'class' => 'guestbook');

To:

$items['guestbook'] = array(
            'title' => t('Guestbook'),
            'value' => $link,
            'class' => 'guestbook');