Interface Node: Make Attr inherit from Node again

[kkdomono]

• whatwg@625a074 Make Attr inherit from Node again
• whatwg@f64b0c8 Put Attr.prototype.ownerElement back; give Attr.prototype.value a setter and an alias named textContent

[kkdomono]

There are already 2 issues about Attr Node, for reference:
#4
#5

[wucongdonglai]

please take care of whatwg@2b76ab8

[yongsheng]

tried in chrome and firefox, below 2 statements return 'true':
document.children[0].attributes[0] instanceof Node
document.children[0].attributes[0] instanceof Attr

will add it in DOM4.1 soon.

[yongsheng]

@linzhiping maybe I can fix this issue since #29 depends on it?

[yongsheng]

@wucongdonglai Attr.nodeValue is deprecated. see https://dom.spec.whatwg.org/#interface-attr.

[yongsheng]

related to #77