Various small fixes and improvements

[tardypad]

These PR introduces various small fixes and improvements described in their own commit message

The main change adds a new setting dj_as_artist to choose whether the DJ name should be used as artist name. The problem encountered is that in some mopidy client (for example I'm using ncmpcpp), the display of the artist clutters the list uselessly.

The default behavior is unchanged (the artist name is set to the DJ one) If you don't like this new config, another option could be to set the DJ name to another track field (like composer or performer) so you keep the DJ information but it is not getting displayed by default.

[coveralls]

Coverage decreased (-1.0%) to 60.274% when pulling 38d42a26f5a67b96fbe926c8d5954dd4f07e7ec0 on tardypad:various_fixes into d5cb6268b2a691c28f7b8845fd0e76b99fad4452 on AlexandrePTJ:develop.

[coveralls]

Coverage decreased (-1.3%) to 60.0% when pulling fb80a6d7796331138a441231027ffa9b252c5ee4 on tardypad:various_fixes into d5cb6268b2a691c28f7b8845fd0e76b99fad4452 on AlexandrePTJ:develop.

[AlexandrePTJ]

@tardypad I am ok with thoses improvements, please resolve conflicts. Thanks.

[coveralls]

Coverage decreased (-0.9%) to 60.0% when pulling 07f386214c043f4ad94c7f20b340edbae7e844ae on tardypad:various_fixes into f92cc163d020150df45c3b61d3ca287de99b27f3 on AlexandrePTJ:develop.

[coveralls]

Coverage decreased (-1.1%) to 59.756% when pulling 0b32eeba9f3de368c6468634d03abdd8756f6474 on tardypad:various_fixes into f92cc163d020150df45c3b61d3ca287de99b27f3 on AlexandrePTJ:develop.

[tardypad]

@AlexandrePTJ I've rebased it on develop. It should be good. Thanks for merging those PRs and for your previous work on this!