Missing tracks plugin

This plugin adds a new command, missing or miss, which finds and lists, for every album in your collection, which tracks are missing. Listing missing files requires one network call to MusicBrainz.

It accepts 'format', 'count', and 'total' configuration directives or command-line options for formatting missing items using the standard format mechanism, displaying missing item counts per album, or simply the total number of missing items.

It does not do any caching, or any other library modification, at this point. This means it requires one single network call to MB per album.

This was originally part of the grooveshark plugin. Since it is difficult to maintain any kind of stability with pygrooveshark, I've refactored the 'missing' functionality out of it and into this pull request, mainly because there appears to be some interest from a few users in this.