v1.3.13 fix issue 24: add 下一张 as next page keyword regexp

I guess this rel="next" link could be picked up automatically:
<link rel="next" href="http://www.douban.com/online/11736105/photo/2188625569/?sortby=time">

But since the standard says a <link> tag can occur only in the head element, I
don't plan to support it now.