Exaile 3.4.2 released (semi-critical bugfix release)

Fixes the following bugs:

• #11 - Exaile did not always update individual files when changed externally
• #12 - Add an option to ‘force rescan’ of files for users with misbehaving external tagger programs
• #13 - GroupTagger plugin: Potential tag loss when using externally modified files

All users are strongly encouraged to upgrade to this release, particularly users of the GroupTagger plugin.