use a new avconv config class to read the avconv command from; use ffmpeg if avconv is not found
(only implemented for metadata-parser so far and not configureable)
use AvconvConfig to read the command for thumbnailCreatorAvConv
actually call the right commad from the config, that was just given.
some documentation that this creator shit won't actually do stuff until you call start
add AvconvConfig to VideoClipCreator and GifCreator
fix a wrong self-reference instead of arg reference in thumbnailcreator