Make file loading more resilient

See (PICARD-605)[http://tickets.musicbrainz.org/browse/PICARD-605]
Add %_multiartist% and fix default renaming.

See http://forums.musicbrainz.org/viewtopic.php?pid=25318
Remove debug print statement
Add NEWS.
Use getall rather than split by ; (bitmap)
Handle NAT tracks with feat. artists.

Add %_primaryartist% and %_primaryartistsort% and use
%_primaryartist% in default naming string.
Revert "Handle NAT tracks with feat. artists."

This reverts commit 0707add3353455473f5ff26a117c7843d42cd4d5.
Switch to using artist rather than artists[0].
Merge NEWS items for compilation and multiartist.
Tweak NEWS (per mineo comment)
Put default script into a constant.
Add non-NAT and NAT tests
Fix default script for NAT tracks
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
Include select module in mac build.

See [Picard-607](http://tickets.musicbrainz.org/browse/PICARD-607) for
requirement.
Merge pull request #337 from Sophist-UK/sophist_include_select_for_py2app

Include select module in mac build.
Raise NotImplementedError
Drop now unused `caa_front_found`
Remove unneeded wrapper methods