Admin Video Mixin list_display

[afk-mario]

Add a callable function to display the video thumbnail on list_display, something like this:

class AdminVideoMixin(object):
    def admin_video_thumbnail(self, obj):
        return u"<img src='%s' />" % VideoBackend(obj.video).thumbnail
    admin_video_thumbnail.allow_tags = True
    admin_video_thumbnail.short_description = u"Video"

[SalahAdDin]

PR is welcome.