Add Transifex settings.
Add transifex-client to requirements.txt.
Ignore compiled message catalogue files (.mo).
Add musicbottle.pot.

Via: `pybabel extract -F babel.cfg --project=MusicBottle -o musicbottle.pot .`
Current Danish translation from Transifex.
Merge branch 'issue-3-import-translations'

Add current Danish translation. There's still stuff needed to be
done, but merging the current status quo to help adoption into
the Weblate system.

Conflicts:
	.gitignore