Support HTML5 <figure> img && minimum image size && positive keywords

1.11.1
1.Support for HTML5 <img> buried in <figure>
2.Add support positive_classes for readability.py. You can use it to find back missing part of original HTML article.
3.Add support for minimum image size.Some imge are useless like 1x1.gif.It effect thumbnail looking.
zh_cn
1.支持解析HMTL5的figure包含的图片
2.支持兴趣关键词，可以用于保留文章某些被readabilty.py删掉的部分。
3.支持最小图片大小。某些网站会在文章正文中加入1x1.gif的水印图片，不仅无用而且干扰thumbnail，一张1*1图片被放大到150*150……

![screenshot_2014_04_02t16_21_04 0800](https://cloud.githubusercontent.com/assets/4038241/2588734/350a8cc2-ba40-11e3-806f-3495691c4de2.png)
![screenshot_2014_04_02t16_21_13 0800](https://cloud.githubusercontent.com/assets/4038241/2588735/350e6acc-ba40-11e3-9861-ef10e605dcbd.png)
![screenshot_2014_04_02t16_21_20 0800](https://cloud.githubusercontent.com/assets/4038241/2588740/56d8521c-ba40-11e3-85a0-8d54b6b1d322.png)
![screenshot_2014_04_02t16_29_58 0800](https://cloud.githubusercontent.com/assets/4038241/2588785/2a75a2be-ba41-11e3-968b-a2158e065a7b.png)