title: Django filter for converting the number of seconds into a readable form
body: <p>I have model:</p>
  
  <pre><code>class Track(models.Model):
      artist = models.CharField(max_length=100)
      title = models.CharField(max_length=100)
      length = models.PositiveSmallIntegerField()
  </code></pre>
  
  <p>where length is the duration of track in seconds</p>
  
  <p>I have template:</p>
  
  <pre><code>{% for track in tracks %}
  {{track.artist}} - {{track.title}} {{track.length}}
  {% endfor %}
  </code></pre>
  
  <p>How can I convert a length in the readable form, ie 320 seconds should be displayed as 5:20, 3770 as 1:02:50?</p>
  
  <p>Forgive sorry for bad english.</p>