不使用图片创建对话气泡 http://rockux.com/archives/%E4%B8%8D%E4%BD%BF%E7%94%A8%E5%9B%BE%E7%89%87%E5%88%9B%E5%BB%BA%E5%AF%B9%E8%AF%9D%E6%B0%94%E6%B3%A1
