Add multi-value variants of mb_artistid and mb_albumartistid.

Tags are now interpreted as lists.
Resolves beetbox#43

Author: JuniorJPDJ

mediafile.py

@@ -1994,18 +1994,27 @@ def update(self, dict):
         StorageStyle('MUSICBRAINZ_ALBUMID'),
         ASFStorageStyle('MusicBrainz/Album Id'),
     )
-    mb_artistid = MediaField(
-        MP3DescStorageStyle(u'MusicBrainz Artist Id'),
-        MP4StorageStyle('----:com.apple.iTunes:MusicBrainz Artist Id'),
-        StorageStyle('MUSICBRAINZ_ARTISTID'),
+    mb_artistids = ListMediaField(
+        MP3ListDescStorageStyle(u'MusicBrainz Artist Id', split_v23=True),
+        MP4ListStorageStyle('----:com.apple.iTunes:MusicBrainz Artist Id'),
+        ListStorageStyle('MUSICBRAINZ_ARTISTID'),
         ASFStorageStyle('MusicBrainz/Artist Id'),
     )
-    mb_albumartistid = MediaField(
-        MP3DescStorageStyle(u'MusicBrainz Album Artist Id'),
-        MP4StorageStyle('----:com.apple.iTunes:MusicBrainz Album Artist Id'),
-        StorageStyle('MUSICBRAINZ_ALBUMARTISTID'),
+    mb_artistid = mb_artistids.single_field()
+
+    mb_albumartistids = ListMediaField(
+        MP3ListDescStorageStyle(
+            u'MusicBrainz Album Artist Id',
+            split_v23=True,
+        ),
+        MP4ListStorageStyle(
+            '----:com.apple.iTunes:MusicBrainz Album Artist Id',
+        ),
+        ListStorageStyle('MUSICBRAINZ_ALBUMARTISTID'),
         ASFStorageStyle('MusicBrainz/Album Artist Id'),
     )
+    mb_albumartistid = mb_albumartistids.single_field()
+
     mb_releasegroupid = MediaField(
         MP3DescStorageStyle(u'MusicBrainz Release Group Id'),
         MP4StorageStyle('----:com.apple.iTunes:MusicBrainz Release Group Id'),

test/test_mediafile.py

@@ -991,8 +991,10 @@ def test_properties_from_readable_fields(self):
 
     def test_known_fields(self):
         fields = list(ReadWriteTestBase.tag_fields)
-        fields.extend(('encoder', 'images', 'genres', 'albumtype', 'artists',
-                       'albumartists', 'url'))
+        fields.extend(
+            ('encoder', 'images', 'genres', 'albumtype', 'artists',
+             'albumartists', 'url', 'mb_artistids', 'mb_albumartistids')
+        )
         assertCountEqual(self, MediaFile.fields(), fields)
 
     def test_fields_in_readable_fields(self):