Convert HTMLElement and friends to classes to support Web Components/Custom elements

Please model HTMLElement and its subclasses as classes instead of interfaces in lib.d.ts to make it possible to subclass them.

My motivation is custom elements and web components. To create a custom element you need to create a subclass of HTMLElement and register it with the browser. Currently, HTMLElement etc are interfaces and can't be subclassed. I've been using a hacked lib.d.ts in my pet TypeScript project which uses web components.

Tutorial and info about how to use custom elements is here: http://www.html5rocks.com/en/tutorials/webcomponents/customelements/

从 lib.d.ts 开始，其中 HTMLElement 及其子类目前被声明为接口。阅读 custom-elements 的动机，并检查周围的 DOM 声明，以了解受影响的层次结构。当这些声明支持为 web components 创建 HTMLElement 和相关元素的子类，同时不破坏现有用法时，即表示完成。