title: nltk NgramModel error
body: <p>I've previously used the <code>NgramModel</code> class in nltk without error. However, I recently updated nltk to <a href="https://github.com/nltk/nltk/blob/develop/ChangeLog" rel="noreferrer">version 3.1</a> and I can no longer find the <code>NgramModel</code> class. When I try to import as usual:  </p>
  
  <p><code>from nltk.model import NgramModel</code></p>
  
  <p>I get the error</p>
  
  <p><code>ImportError: No module named 'nltk.model'</code>.</p>
  
  <p>Has <code>NgramModel</code> or the <code>model</code> module been recently replaced?</p>