require python-musicbrainz-ngs 0.2 or newer

This came up because version 0.1 (which is available in some Debian repos)
apparently lack `set_hostname`.
Implement BeetsPlugin.item_fields()

This uses custom StorageStyle fields following beets' other custom
fields' convention of prefixing a 'namespace' in description
fetchart: remote_priority config option
Merge branch 'master' of https://github.com/sampsyo/beets into echonest_plugin