title: Pandas sort on index throws "unhashable type: 'list'" error
body: <p>I create my dataset as follows (intentionally out of order):</p>
  
  <pre><code>pa = pd.MultiIndex.from_tuples([('1', '2'),('2', '1'),('1', '4'),('1', '5'),('1', '3'),('2', '2'),('2', '3'),('3', '1'),('3', '2'),('1', '1')]
                             , names=['batch', 'run'])
  yld=pd.DataFrame(np.random.randn(10,1),index=pa,columns=['yield'])
  </code></pre>
  
  <p>when I try to a sort on the <code>batch</code> &amp; <code>run</code> index columns:</p>
  
  <pre><code>yld.sort_index(['batch','run'])
  </code></pre>
  
  <p>I get a <code>TypeError: unhashable type: 'list'</code></p>
  
  <p>I don't have a clue what is going wrong..</p>