Embedart TestCase
Test converter embeds album art
Refactor embedart to work at item level

Embedding images now triggers the `*_write` plugin events. This
allows *beets-check* to update the checksum. See the [beets-check
issue][1].

[1]: https://github.com/geigerzaehler/beets-check/issues/7
Merge pull request #802 from geigerzaehler/embed

Merge pull request #802