title: Embedded objects in fixtures raise "unable to find fixtures" errors
body: <p>I'm using ember-data's fixtures adapter extensively to provide a lot of set-up data in my app. For one model ("Structure"), there is an array of embedded related objects ("Overlays"). When I load a Structure instance from the store, then query its <code>overlays</code> property, I get this error:</p>
  
  <pre><code>Uncaught Error: assertion failed: Unable to find fixtures for model type App.Overlay
  </code></pre>
  
  <p>Both models are defined as follows (with a few other fields/relationships removed for clarity):</p>
  
  <pre><code>App.Structure = DS.Model.extend({
    // How this appears in lists
    name: DS.attr('string'),
    // Color to show this overlay with
    color: DS.attr('string'),
    // hash of overlay paths
    overlays: DS.hasMany('App.Overlay', { embedded: true })
  });
  
  App.Overlay = DS.Model.extend({
    view: DS.attr('string'),
    path: DS.attr('string')
  });
  </code></pre>
  
  <p>I only need one fixture to demonstrate:</p>
  
  <pre><code>App.Structure.FIXTURES = [{
      "id": 0,
      "name": "Test Structure",
      "color": "#cc0033",
      "overlays": [{
          'view': 'Isometric',
          'path': "M17.5,19.508V8.626h-3.999v10.881c-1.404,0.727-2.375,2.178-2.375,3.869c0,2.416,1.959,4.375,4.375,4.375s4.375-1.959,4.375-4.375C19.876,21.686,18.905,20.234,17.5,19.508zM20.5,5.249c0-2.757-2.244-5-5.001-5s-4.998,2.244-4.998,5v12.726c-1.497,1.373-2.376,3.314-2.376,5.4c0,4.066,3.31,7.377,7.376,7.377s7.374-3.311,7.374-7.377c0-2.086-0.878-4.029-2.375-5.402V5.249zM20.875,23.377c0,2.963-2.41,5.373-5.375,5.373c-2.962,0-5.373-2.41-5.373-5.373c0-1.795,0.896-3.443,2.376-4.438V5.251c0-1.654,1.343-3,2.997-3s3,1.345,3,3v13.688C19.979,19.934,20.875,21.582,20.875,23.377zM22.084,8.626l4.5,2.598V6.029L22.084,8.626z"
      }]
  }];
  </code></pre>
  
  <p>With those, I can load a Structure:</p>
  
  <pre><code>&gt; var structure = App.fixtureStore.find(App.Structure, 0);
  &gt; structure.get('name');
  "Test Structure"
  </code></pre>
  
  <p>But if I query its overlay, things go squirrely:</p>
  
  <pre><code>&gt; structure.get('overlays');
  Uncaught Error: assertion failed: Unable to find fixtures for model type App.Overlay
  </code></pre>
  
  <p>What's up? Do I need to declare fixtures for this model, even if they're empty?</p>
  
  <p><a href="http://jsfiddle.net/pjmorse/hA6VH/" rel="nofollow">Here is a jsfiddle showing the error</a>.</p>