Use Media.Piece in Playlist , fixes #49 (2)

This is basically #92 without the _id generation. I don't know why it wasn't working before.

When adding a new Media to a List we instance a Piece with a
hopefully unique _id within the List and a 'checksum' field pointing
to the Media's md5.

Caveats:

    needs a collection rescan

Fixes #49