0.5.3 release
    
    * Bugfix: Cascade top document's level to subframes with no explicitly assigned level. (workaround
    	for Github issue #133).
    * Bugfix: Improve IPv6 handling in NBS
    * Bugfix (Chromium-based browsers): Fix per-domain level options/settings page (Github issue #147)
    * Do not display NBS notifactions when accessing 0.0.0.0 and :: (workaround
    	for Github issue #125)
    * Improve NBS description in the option/settings page.
    * Display level names in the pop up to improve usability.