Split cover art providers to their own classes and files

- add extension point so one can write a cover art provider plugin
- minor changes to ease the move
Introduce CoverArtProvider.match_url_relations() to reduce code redundancy
Raise NotImplementedError
Drop now unused `caa_front_found`
Remove unneeded wrapper methods
Clarify queue_downloads() return values
Fix up `CoverArtProvider.match_url_relations` description
Describe CoverArtProvider class
Merge pull request #336 from zas/cover_art_providers

Split cover art providers to their own classes and files