title: With json4s how to add field on-the-fly during serialization
body: <p>I have a case class which has several <code>DateTime</code> fields. While using <code>json4s</code> to serialize it I want to serialize each of these fields as 2 separated fields - one in formatted datetime string and the other one in unix timestamp.</p>
  
  <p>So for example the case class is:</p>
  
  <pre><code>case class Event {
      name: String,
      start: DateTime
  }
  </code></pre>
  
  <p>For an object:</p>
  
  <pre><code>val event = Event("foo", DateTime.now)
  </code></pre>
  
  <p>I want the serialized json string to be:</p>
  
  <pre><code>{
      "name": "foo",
      "start": "2014-04-01T09:00:00+0000",
      "startUnixtime": 1396342800
  }
  </code></pre>
  
  <p>I've tried <code>FieldSerializer</code> and <code>CustomSerializer</code> but couldn't get it work.</p>