initial version of the mbsync plugin

this plugin provides a faster way to query new metadata from
musicbrainz. (instead of having to 're-import' the files)
Currently it lacks all forms of documentation and will only work for
album queries. not really tested so far so be careful
mbsync: write metadata to files...
mbsync: documentation
mbsync: add support for singletons

I can't really guarantee this works right now since I have no singletons
in my collection to test it
mbsync: update docs
mbsync: a little refactoring