理解python metaclass使用技巧与应用场景分析
参考：
- decorator与metaclass: http://jfine-python-classes.readthedocs.org/en/latest/index.html
- 深入理解python中的metaclass: http://blog.jobbole.com/21351/
- 廖雪峰：http://www.liaoxuefeng.com/wiki/001374738125095c955c1e6d8bb493182103fac9270762a000/001386820064557c69858840b4c48d2b8411bc2ea9099ba000
- stackoverflow:http://stackoverflow.com/questions/100003/what-is-a-metaclass-in-python
- monkey_patch:http://stackoverflow.com/questions/5626193/what-is-a-monkey-patch
- python高级技巧之 函数式编程： http://blog.jobbole.com/66895/
- python高级技巧之 metaclass： http://blog.jobbole.com/67748/
- http://jianpx.iteye.com/blog/908121
- http://py.windrunner.info/design-patterns/singleton.html