Add two way data binding.

sky/framework/sky-element/sky-binder.sky

 <!--
 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 -->
 <import src="observe.sky" as="observe" />
 
 <script>
 var stagingDocument = new Document();
 
@@ skipping 142 matching lines @@
       }));
     } else if (name == 'style' || name == 'class') {
       updateAttribute(node, name, observable.open(function(value) {
         updateAttribute(node, name, value);
       }));
     } else {
       node[name] = observable.open(function(value) {
         node[name] = value;
       });
     }
+    if (typeof node.addPropertyBinding == 'function')
+      node.addPropertyBinding(this.name, observable);
     return observable;
   }
 }
 
 function parsePropertyDirective(value, property) {
   if (!value || !value.length)
     return;
 
   var result;
   var offset = 0;
@@ skipping 347 matching lines @@
     iterators.delete(this.template);
     this.closed = true;
   }
 }
 
 module.exports = {
   createInstance: createInstance,
   registerElement: registerElement,
 };
 </script>