video_/audio_xml won't crash anymore when no videos are available
TMDB certification(MPAA) considers now the language from config
Improved actor XML
Code cleanup
Responsibility of a method adapted
Refactored audio XML generation
Refactored video XML generation
Improved XML creation for movies

NFO XML is now created with lxml module
Improved XML creation for TV shows

NFO XML is now created by lxml.
The method that creates the NFO is separated from the TvdbApi class - belongs to #3