allow MP3StorageStyle.fetch() to fallback to url attribute

[lukeIam]

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).

[semohr]

This looks like a more general issue affecting everything in 4.3.1 of the spec

The proposed fix will probably work, but it feels a bit hacky. What do you think about instead adding a comment and checking explicitly for the frame type? In this case, checking for a UrlFrame should be sufficient.

Do you have ideas on how we could test this to make sure it behaves as expected?

And apologies for the long delay, mediafile has been flying under the radar for many of us in the beets team.