title: django json serializer does not implement geojson
body: <p>I am in the process of building a website on geodjango.  On the front end I have a map on openlayers and I want to be able to fire ajax queries at some django url and get back geojson.</p>
  
  <p>The problem is that the standard django json serializer doesn't do geojson.  So I have a model like:</p>
  
  <pre><code>class Asset (models.Model):
  objects = models.GeoManager()
  
  url_name = models.CharField(max_length=200)
  name = models.CharField(max_length=200)
  point = models.PointField(srid=4326)
  
  def __unicode__(self):
      return self.name
  </code></pre>
  
  <p>And I want to create a view:</p>
  
  <pre><code>def geojson_query(request):
  
  #geographic query
  
  ...
  
  assets = Asset.objects.all().filter(point__contained=bb_4326)
  json = serializers.serialize("json", assets)
  return HttpResponse(json)
  </code></pre>
  
  <p>But then I get back a response that looks more like this:</p>
  
  <pre><code>"url_name": "Upper_Svaneti", 
  "name": "Upper Svaneti", 
  "point": "POINT (43.0113899999999987 42.9163899999999998)"
  </code></pre>
  
  <p>Whereas I want something that looks like this:</p>
  
  <pre><code>"url_name": "Upper_Svaneti", 
  "name": "Upper Svaneti", 
  "geometry": {
  "type": "Point", 
  "coordinates": [43.0113899999999987 42.9163899999999998]
  }
  </code></pre>
  
  <p>So I guess my question is, what is the best way to implement a geojson serializer in django without totally rolling my own?  And if it comes to rolling my own, where do I start?</p>