auto merge of #983 : saneyuki/servo/getid, r=jdm

This has not implemented these features yet:
* Update hashmap which is used for `getElementById()` when updating element attributes.
* Keep node order in tree.

However, this has the quality of that we can use this method for testcases.

src/components/script/dom/element.rs

@@ -170,6 +170,9 @@ impl<'self> Element {
                     null_str_as_empty_ref(raw_value)));
         }
 
+        // TODO: update owner document's id hashmap for `document.getElementById()`
+        //       if `name` == "id".
+
         //XXXjdm We really need something like a vtable so we can call AfterSetAttr.
         //       This hardcoding is awful.
         match abstract_self.type_id() {