first draft of polymer element tutorial

src/site/docs/tutorials/fetchdata/index.markdown

 ---
 layout: default
 title: "Fetch Data Dynamically"
 description: "Use HttpRequest to fetch data from a file or a server."
 has-permalinks: true
 tutorial:
   id: fetchdata
 next: forms/
 next-title: "Get Input from a Form"
-prev: custom-elements/
-prev-title: "Define a Custom DOM Tag"
+prev: polymer-intro/
+prev-title: "Define a Custom Element"
 ---
 
 {% capture whats_the_point %}
 
 * Data on the web is often formatted in JSON.
 * JSON is text based and human readable.
 * The dart:json library provides support for JSON.
 * Use HttpRequest to dynamically load data.
 
 {% endcapture %}
@@ skipping 444 matching lines @@
 
 Check out Chris Buckett's article,
 <a href="/articles/json-web-service/"
    target="_blank">Using Dart with JSON Web Services</a>,
 for more information and an example with source code for both
 client and server programs.
 
 {% endcapture %}
 
 {% include tutorial.html %}