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
Fix regression, save only ONE front image when "save_only_front_images_to_tags" is set

This regression was introduced during code rewrite, option name is misleading.

See https://github.com/musicbrainz/picard/pull/116
See PICARD-350 (which wasn't closed)
Only download one front image if no more is needed according to options
Merge pull request #333 from zas/fix_front_image_only

Fix regression, save only ONE front image when "save_only_front_images_t...
Include socket module in py2app

See [Picard-607](http://tickets.musicbrainz.org/browse/PICARD-607) for
requirement.
Merge pull request #335 from Sophist-UK/sophist_include_socket_for_py2app

Include socket module in py2app
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.