Fix bug passing language to VideoLog.update_video_log. Include language in __unicode__ output.
**DO NOT include new fields (i.e. fields with minversion) to be used in _hashable_representation.**  Migrate _hashable_fields to a class function.
Added documentation for signed_version
Merge pull request #807 from bcipolli/hashable_fields_minversion

Do not include new fields in _hashable_representation