Add support for splitting of ID3v2.3 multi-value tags

fixes rest of problems in beetbox#43

Shouldn't be enabled on tags like `artists` or other without confidence that values will not contain separator (eg. `artists: ['AC/DC']`).
Format of mbid is known and I'm sure we can enable it here.

When solving beetbox#21 this separator value in `el.split('/')` also should be taken into account.

mediafile.py

@@ -893,14 +893,19 @@ def delete(self, mutagen_file):
 
 
 class MP3ListDescStorageStyle(MP3DescStorageStyle, ListStorageStyle):
-    def __init__(self, desc=u'', key='TXXX', **kwargs):
-        super(MP3ListDescStorageStyle, self).__init__(desc=desc, key=key,
-                                                      **kwargs)
+    def __init__(self, desc=u'', key='TXXX', split_v23=False, **kwargs):
+        self.split_v23 = split_v23
+        super(MP3ListDescStorageStyle, self).__init__(
+            desc=desc, key=key, **kwargs
+        )
 
     def fetch(self, mutagen_file):
         for frame in mutagen_file.tags.getall(self.key):
             if frame.desc.lower() == self.description.lower():
-                return frame.text
+                if mutagen_file.tags.version == (2, 3, 0) and self.split_v23:
+                    return sum([el.split('/') for el in frame.text], start=[])
+                else:
+                    return frame.text
         return []
 
     def store(self, mutagen_file, values):
@@ -1988,15 +1993,16 @@ def update(self, dict):
         ASFStorageStyle('MusicBrainz/Album Id'),
     )
     mb_artistid = ListMediaField(
-        MP3ListDescStorageStyle(u'MusicBrainz Artist Id'),
+        MP3ListDescStorageStyle(u'MusicBrainz Artist Id', split_v23=True),
         MP4ListStorageStyle('----:com.apple.iTunes:MusicBrainz Artist Id'),
         ListStorageStyle('MUSICBRAINZ_ARTISTID'),
         ASFStorageStyle('MusicBrainz/Artist Id'),
     )
     mb_albumartistid = ListMediaField(
-        MP3ListDescStorageStyle(u'MusicBrainz Album Artist Id'),
+        MP3ListDescStorageStyle(u'MusicBrainz Album Artist Id', split_v23=True),
         MP4ListStorageStyle(
-            '----:com.apple.iTunes:MusicBrainz Album Artist Id'),
+            '----:com.apple.iTunes:MusicBrainz Album Artist Id'
+        ),
         ListStorageStyle('MUSICBRAINZ_ALBUMARTISTID'),
         ASFStorageStyle('MusicBrainz/Album Artist Id'),
     )