Implement document.compatMode

Spec:
http://dom.spec.whatwg.org/#dom-document-compatmode

This is a sub-task for #1428.

src/components/script/html/hubbub_html_parser.rs

@@ -445,6 +445,7 @@ pub fn parse_html(cx: *JSContext,
         },
         set_quirks_mode: |_mode| {
             debug!("set quirks mode");
+            document.mut_document().set_quirks_mode(_mode);
         },
         encoding_change: |_encname| {
             debug!("encoding change");