Added support for polymorphic associations.

```javascript
App.User = DS.Model.extend({
 messages: DS.hasMany(App.Message, {polymorphic: true})
});

App.Message = DS.Model.extend({
  created_at: DS.attr('date'),
  user: DS.belongsTo(App.User)
});

App.Post = App.Message.extend({
  title: DS.attr('string')
});

App.Comment = App.Message.extend({
  body: DS.attr('string'),
  post: DS.belongsTo(App.Post)
});
```
* You can add support for embedded polymorphic associations by overriding`addHasMany` of the serializer.

* `BaseClass.find()` isn't supported yet and can be the subject of another PR.