Implement document.URL & document.documentURI

Specs:
http://dom.spec.whatwg.org/#dom-document-url
http://dom.spec.whatwg.org/#dom-document-documenturi

This is a sub-task for #1428.
auto merge of #1583 : brunoabinader/servo/document-url, r=Ms2ger

Specs:
http://dom.spec.whatwg.org/#dom-document-url
http://dom.spec.whatwg.org/#dom-document-documenturi

This is a sub-task for #1428.