Add sky-drawer to the framework

sky/framework/sky-box.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="sky-element/sky-element.sky" as="SkyElement" />
 
 <sky-element name="sky-box" attributes="title:string">
 <template>
   <style>
@@ skipping 10 matching lines @@
       padding: 8px 8px 4px 8px;
     }
     .content {
       padding: 4px 8px 8px 8px;
     }
     div {
       flex: 1;
     }
   </style>
   <div class="title">{{ title }}</div>
-  <div class="content"><content></content></div>
+  <div class="content"><content/></div>
 </template>
 <script>
 module.exports = class extends SkyElement {
 }.register();
 </script>
 </sky-element>