doc: http://open.youku.com/docs/doc?id=71

def find_videos_by_playlist(self, playlist_id, page=1, count=20):
        """doc: http://open.youku.com/docs/doc?id=71
        """
        url = 'https://openapi.youku.com/v2/playlists/videos.json'
        params = {
            'client_id': self.client_id,
            'playlist_id': playlist_id,
            'page': page,
            'count': count
        }
        r = requests.get(url, params=params)
        check_error(r)
        return r.json()