allow MP3StorageStyle.fetch() to fallback to url attribute #83
  Add this suggestion to a batch that can be applied as a single commit.
  This suggestion is invalid because no changes were made to the code.
  Suggestions cannot be applied while the pull request is closed.
  Suggestions cannot be applied while viewing a subset of changes.
  Only one suggestion per line can be applied in a batch.
  Add this suggestion to a batch that can be applied as a single commit.
  Applying suggestions on deleted lines is not supported.
  You must change the existing code in this line in order to create a valid suggestion.
  Outdated suggestions cannot be applied.
  This suggestion has been applied or marked resolved.
  Suggestions cannot be applied from pending reviews.
  Suggestions cannot be applied on multi-line comments.
  Suggestions cannot be applied while the pull request is queued to merge.
  Suggestion cannot be applied right now. Please check back later.
  
    
  
    
In the issue Neurrone/beets-audible#71 we get a crash if the "WOAF" element does not have an "text" but an "url" attribute.
I added a fallback to also use the "url" attribute on
MP3StorageStyle.fetch().But I'm not an mp3 expert at all - please double check (maybe additional changes at other places are required).