first draft of polymer element tutorial

src/site/docs/tutorials/custom-elements/index.markdown

 ---
 layout: default
 title: "Define a Custom DOM Tag"
 description: "Define a custom DOM element tag with help from the Web UI package"
 has-permalinks: true
 tutorial:
   id: web-components
-next: fetchdata/
-next-title: "Fetch Data Dynamically"
+next: polymer-intro/
+next-title: "Polymer"
 prev: templates/
 prev-title: "Use Templates"
 ---
 
 {% capture whats_the_point %}
 
 * Make your own DOM element tags with the <element> tag.
 * A custom element requires a template and can have a script.
 * Use data binding to connect Dart variables to content.
 * Directly attach event handlers in HTML.
@@ skipping 384 matching lines @@
   The Web UI package contains several examples,
   including a version of
   <a href="https://github.com/dart-lang/web-ui/tree/master/example/todomvc"
      target="_blank">TodoMVC</a>.
 </li>
 </ul>
 
 {% endcapture %}
 
 {% include tutorial.html %}