Fetch classical-related tags from MusicBrainz

Please sync the multi-valued tags
- PERFORMER
- CONDUCTOR
- ENSEMBLE
- ARRANGER
- COMPOSER
- etc.
from MB.

Ideally, this would include what they're credited for. Using this [Beethoven work](http://musicbrainz.org/recording/8869b7fd-3df3-4351-8fdb-321fc757244f) as an example, I'd hope to see something like:

```
PERFORMER=Norman Scott (bass vocals)
PERFORMER=Robert Shaw Choral (choir vocals)
CONDUCTOR=Robert Shaw (chorus master)
CONDUCTOR=Arturo Toscanini
PERFORMER=Nan Merriman (mezzo-soprano vocals)
ENSEMBLE=NBC Symphony Orchestra
PERFORMER=Eileen Farrell (soprano vocals)
PERFORMER=Jan Peerce (tenor vocals)
COMPOSER=Ludwig van Beethoven
```

This is inspired from the recommendations on http://age.hobba.nl/audio/mirroredpages/ogg-tagging.html , which builds on the [Ogg Vorbis header specification](http://www.xiph.org/vorbis/doc/v-comment.html). In particular, the header specification says "Field names are not required to be unique (occur once) within a comment header. As an example, assume a track was recorded by three well know artists; the following is permissible, and encouraged:"

```
ARTIST=Dizzy Gillespie
ARTIST=Sonny Rollins
ARTIST=Sonny Stitt
```

This depends on [issue 505](https://github.com/sampsyo/beets/issues/505).