Improvements to ASF tags:

See ticket in http://tickets.musicbrainz.org/browse/PICARD-648

* Distingiush between WM/OriginalReleaseTime and WM/OriginalReleaseYear
* Save encodersettings to WM/EncodingSettings
* Save website to WM/AuthorURL
* Support WM/PartOfSet possibly containing totaldiscs