convert: copy album art when copying or transcoding.

Only implemented to work when matching albums instead of tracks (-a option).
Disabled by default in the configuration of 'convert'.

fix indentation.

Fix Travis nagging over indentation (hopefully).

Finally pep8 conforming.
convert: update documentation and add a trailing comma.
Merge pull request #1055 from alderz/convert_copy_album_art

convert: copy album art when copying or transcoding.