参考文献：
1、Angus Croll.Understanding JavaScript Prototypes.
2、Vjeux.Javascript – How Prototypal Inheritance really works.
3、Dmitry Soshnikov.JavaScript. The core.
还没能完全理解，多看几遍再整理。
__proto__与prototype
原文：http://www.cnblogs.com/yangfang228/p/6442472.html
