updates

Hi tatewake, I've forked an updated version of your plugin and was wondering if you were interested in pulling the changes.

The changes:
  -now respects $conf['savedir'] and DOKU_CONF being elsewhere than default locations (as is the case in my wiki where .htaccess doesn't work to prevent direct data access).
  -GNU Tar 'slow' mode is much faster and invokes tar only one time, through the use of a manifest file.
  -backups are no longer located in media root, but in a customizable namespace -> allows ACL to prevent prying eyes from downloading old backups.
  -view and delete old backups from admin.php