Fix #82 - use the native DOM node type that we are extending.
In practice, it means `<x-foo>` becomes `<base-tag is="x-foo">`.

Pros:
* extends tag now does something
* custom tags render correctly and host node has the right APIs
* makes our custom-tags less leaky

Cons:
* Query and CSS selectors become more leaky, need to use base-tag[is=custom-tag]

(where "leaky" means "leaky abstraction")

Review URL: https://chromiumcodereview.appspot.com//13607004