title: react-router browserHistory removeListen?
body: <p>I subscribe to route changes with</p>
  
  <pre><code>browserHistory.listen(fn)
  </code></pre>
  
  <p>but how can I unsubscribe?</p>
  
  <p>I have tried assigning to <code>listen</code> <code>null</code>, but it complains about not being a function.</p>