Follow ReferenceFields in EmbeddedDocuments with select_related

For the following structure:

    class Playlist(Document):
        items = ListField(EmbeddedDocumentField("PlaylistItem"))

    class PlaylistItem(EmbeddedDocument):
        song = ReferenceField("Song")

    class Song(Document):
        title = StringField()

this patch prevents the N+1 queries otherwise required to fetch all
the `Song` instances referenced by all the `PlaylistItem`s.