title: oneOf in Swagger schema does not work
body: <p>I want to define <code>PaymentMethod</code> as below. Is oneOf supported in swagger.yaml?</p>
  
  <pre class="lang-yaml prettyprint-override"><code>PaymentMethod:
        oneOf:
          - $ref: '#/definitions/NewPaymentMethod'
          - $ref: '#/definitions/ExistPaymentMethod'
  </code></pre>
  
  <p>The <code>ExistPaymentMethod</code> will have just id, and <code>cardNumber</code> where <code>NewPaymentMethod</code> will have no <code>id</code>, but all other details, e.g. <code>cardNumber</code>, <code>cardholderName</code>, <code>cardholderAddress</code> etc.</p>