title: add created_at and updated_at fields to mongoose schemas
body: <p>Is there a way to add created_at and <code>updated_at</code> fields to a mongoose schema, without having to pass them in everytime new <code>MyModel()</code> is called?</p>
  <p>The <code>created_at</code> field would be a date and only added when a document is created.
  The <code>updated_at</code> field would be updated with new date whenever <code>save()</code> is called on a document.</p>
  <p>I have tried this in my schema, but the field does not show up unless I explicitly add it:</p>
  <pre><code>var ItemSchema = new Schema({
      name    : { type: String, required: true, trim: true },
      created_at    : { type: Date, required: true, default: Date.now }
  });
  </code></pre>