Fix and extend artists tag ...

1. artists tag is incorrectly added to a release - now added only to a
track
2. add _albumartists and _albumartists_sort for use in scripts and
plugins.
Fix sub-menu capability for plugins...

1. xrange(x, y) goes from x to y-1. If len(MENU) is 1 this is never
called. So add 1 to get correct range.

2. Exception handling is typically slower and code is less clear than if
we just test for the key existing.
Minor style improvement
Add NEWS.txt
Show id3 version in info - resolves PICARD-218
Add NEWS
Tweak NEWS
Merge pull request #268 from Sophist-UK/sophist_fix_and_extend_artists_tag

Fix and extend artists tag ...
Fix typo in commit d65ecf4bdd2269632a53b1fed5606ef22860e974
Fix broken test_mbxml

broken by commit 69228485452dfb7f8e665ea8996e75b9db66234a
Merge pull request #274 from Sophist-UK/sophist_fix_typo_in_#251

Fix typo in #251
Merge pull request #278 from Sophist-UK/sophist_fix_test_mbxml_after_#268

Fix broken test_mbxml
Get rid of 'if logdebug: logdebug(...)' pattern using a noop function

Suggested by @Mineo in https://github.com/musicbrainz/picard/pull/254/files#r11290500
Fix instance where QStringList is now a list (post-fa66e9f)
Merge pull request #280 from zas/i18n_noop

Get rid of 'if logdebug: logdebug(...)' pattern using a noop function
Merge pull request #270 from Sophist-UK/sophist_show_id3_version

Show id3 version in info - resolves PICARD-218
Merge pull request #269 from Sophist-UK/sophist_fix_plugin_MENU_capability

Fix sub-menu capability for plugins...
Merge remote-tracking branch 'upstream/master' into cover_art_types

Conflicts:
	picard/i18n.py
Rework `ugettext_ctxt`

There is no need to strip whitespaces, code was borrowed from django, but this part
doesn't apply here.