封装:http://www.ruanyifeng.com/blog/2010/05/object-oriented_javascript_encapsulation.html
构造函数的继承:http://www.ruanyifeng.com/blog/2010/05/object-oriented_javascript_inheritance.html
非构造函数的继承:http://www.ruanyifeng.com/blog/2010/05/object-oriented_javascript_inheritance_continued.html