title: Threejs blender exporter exports in wrong format
body: <p>I'm trying to export a blender model to threejs json with the help of the provided <a href="https://github.com/mrdoob/three.js/tree/master/utils/exporters/blender" rel="noreferrer">blender exporter</a>. On parsing the json file I get an error:</p>
  
  <pre><code>Uncaught TypeError: Cannot read property 'length' of undefined
  </code></pre>
  
  <p>The json file looks as following (ommiting vertices and faces):</p>
  
  <pre><code>{
      "textures": [],
      "metadata": {
          "version": 4.3,
          "sourceFile": "UV-World.blend",
          "type": "Object",
          "generator": "io_three"
      },
      "images": [],
      "materials": [],
      "object": {
          "matrix": [1,0,0,0,0,1,0,0,0,0,1,0,0,0,0,1],
          "children": [{
              "type": "Mesh",
              "name": "Kugel",
              "uuid": "5051D840-9982-4F9A-8CFF-36303B38E091",
              "position": [0.0,0.0,0.0],
              "rotation": [-1.5707964897155762,0.0,0.0],
              "scale": [1.0,0.9999999403953552,0.9999999403953552],
              "visible": true,
              "castShadow": true,
              "receiveShadow": true,
              "geometry": "DEAE2CE2-9824-4F91-BE4C-1C80BEDC566A"
          }],
          "type": "Scene",
          "uuid": "8C543616-2AB0-4D1D-B76D-549E32A0E468"
      },
      "geometries": [{
          "type": "Geometry",
          "data": {
              "name": "KugelGeometry",
              "metadata": {
                  "version": 3,
                  "faces": 512,
                  "vertices": 482,
                  "generator": "io_three"
              },
              "vertices": [...],
              "faces": [...]
          },
          "uuid": "DEAE2CE2-9824-4F91-BE4C-1C80BEDC566A"
      }]
  }
  </code></pre>
  
  <p>This seems to be not in the <a href="https://github.com/mrdoob/three.js/wiki/JSON-Model-format-3" rel="noreferrer">format that threejs expects</a> it to be. If I manually change the json to be the following threejs parses and displays the model as expected.</p>
  
  <pre><code>{
      "scale": 1,
      "vertices": [...],
      "faces": [...]
  }
  </code></pre>
  
  <p>Exporting and importing to/from obj works as expected.</p>
  
  <p>As I could not find anyone also having this problem, I guess it's a usage error. I'm new to blender, so maybe I did something wrong there? I'm using Blender 2.73a and threejs just cloned from the repository.</p>