fix and improve metadata handling

[nyo]

changelog:

• [x] fixed a bug where download of songs with no description did not work (closes https://github.com/nyo/scdl/issues/7)
• [x] added support for publisher_metadata which is metadata entered by the person uploading the music (= more reliable)
  • now uses publisher_metadata.artist before user.username for artist name
  • now uses publisher_metadata.release_title before title for song title
  • now uses release_date before created_at for album release year
  • now uses publisher_metadata.writer_composer for song composer name