Larger clode clean up

Now using 2 classes for a PictureList and PictureRef. Both encapsulate how a PictureList (tag or event)
is serialized (or rather, just expresssed as a simple dict).

    pl = PictureList("name")
    pl.add_picture(p)
    pl.get_dict()

    pref = PictureRef()
    pref.get_dict(picture, with_exif)

    # etc