title: Reverting from multiindex to single index dataframe in pandas
body: <pre><code>                       NI
  YEAR MONTH datetime        
  2000 1     2000-01-01   NaN
             2000-01-02   NaN
             2000-01-03   NaN
             2000-01-04   NaN
             2000-01-05   NaN
  </code></pre>
  
  <p>In the dataframe above, I have a multilevel index consisting of the columns:</p>
  
  <pre><code>names=[u'YEAR', u'MONTH', u'datetime']
  </code></pre>
  
  <p>How do I revert to a dataframe with 'datetime' as index and 'YEAR' and 'MONTH' as normal columns?</p>