Sophist add itunes composer support

This PR includes the changes introduced by PR-224 - but since they have the same commit hashes, hopefully this will still merge OK.

This enhancement fills out the iTunes non-standard tag enhancements by supporting Composer Sort (TSOC) - I have also added support for ASF based files (and I think mp4s already have support).

When this is merged I will update the wiki page.

Functionality requested in http://forums.musicbrainz.org/viewtopic.php?pid=25180 - I thought it made sense to fill out the compatibility with iTunes unofficial id3v23 extensions by creating this.