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
Use new translate_caa_type() to handle caa type translations
Merge Image() and CoverArtImage()

- Info dialog Artwork tab now displays types and comment for cover art images
- a tooltip gives info about the source (url, caa url, local file, tag, ...)
Make exit cleanup process more generic
Drop refcount/__del__ mess, make code much more reliable and simpler
Introduce image_info() to extract width, height from image data
Use real image files to test formats
Fix deprecated exception.message
imageinfo: add extension to result