Bug 907929 - vorbis art cover

Currently on OggVorbis. Need a trial run.
`prop.start` and `prop.end` aren't necessary here, nor is 'prop.type` from earlier. I'd recommend doing something like this:

```
metadata.picture = {flavor: 'unsynced', blob = aBlob.slice(...)};
```

You could call `unsynced` something else, like  `encoded`, but this is basically the same case as unsynced ID3 artwork, so I'd prefer we just use `unsycned` (even though it's not strictly accurate).