Release 0.4

Signed-off-by: Alastair Porter <alastair@porter.net.nz>
Prepare for 0.5 development

Signed-off-by: Alastair Porter <alastair@porter.net.nz>
musicbrainz.py: Fix undefined name in submit_barcodes

Signed-off-by: Simon Chopin <chopin.simon@gmail.com>
Merge remote-tracking branch 'laarmen/fix-undefined-name'

Signed-off-by: Johannes Dewender <github@JonnyJD.net>