add datetime to the list of known attributes

as per http://www.whatwg.org/specs/web-apps/current-work/multipage/text-level-semantics.html#attr-time-datetime

src/dom/DefaultDOMPropertyConfig.js

@@ -48,6 +48,7 @@ var DefaultDOMPropertyConfig = {
     contentEditable: null,
     controls: MUST_USE_PROPERTY | HAS_BOOLEAN_VALUE,
     data: null, // For `<object />` acts as `src`.
+    datetime: null,
     dir: null,
     disabled: MUST_USE_PROPERTY | HAS_BOOLEAN_VALUE,
     draggable: null,