title: Retrofit GSON serialize Date from json string into long or java.lang.Long
body: <p>I am using the Retrofit library for my REST calls.
  The JSON that is coming in looks like this.</p>
  
  <pre><code>{
      "created_at": "2013-07-16T22:52:36Z",
  } 
  </code></pre>
  
  <p>How can I tell Retrofit or Gson to convert this into <strong>long</strong>?</p>