tracker-extract: Add Gibest hash calculation to GStreamer

Add Gibest extract to GStreamer extractor, as used in the
OpenSubtitles service:
http://trac.opensubtitles.org/projects/opensubtitles/wiki/HashSourceCodes

extract_gibest_hash() written by Carlos Garnacho <carlosg@gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=726640