title: Unexpected result of DataFrame sort
body: <p>When I sort my DataFrame using several columns (<code>['Symbol','Year','Month','Day']</code>) the resulting DataFrame is sorted by <code>Symbol &gt; Year &gt; Month</code> but <strong>not</strong> by <code>Day</code>:</p>
  
  <pre><code>In [1]: df = pd.DataFrame({'Symbol': {79: 'F', 81: 'F', 82: 'F', 83: 'F', 84: 'F', 85: 'F', 86: 'F', 87: 'F', 89: 'F'}, 'Shares': {79: 100, 81: 100, 82: 100, 83: 100, 84: 100, 85: 100, 86: 100, 87: 100, 89: 100}, 'Month': {79: '08', 81: '08', 82: '08', 83: '08', 84: '08', 85: '08', 86: '08', 87: '08', 89: '09'}, 'Year': {79: '2008', 81: '2008', 82: '2008', 83: '2008', 84: '2008', 85: '2008', 86: '2008', 87: '2008', 89: '2008'}, 'Action': {79: 'Sell', 81: 'Sell', 82: 'Buy', 83: 'Sell', 84: 'Buy', 85: 'Sell', 86: 'Buy', 87: 'Sell', 89: 'Sell'}, 'Day': {79: 2L, 81: 4L, 82: '06', 83: 11L, 84: '13', 85: 18L, 86: '18', 87: 23L, 89: 22L}})
  
  In [2]: df
  Out[2]:
     Action Day Month  Shares Symbol  Year
  79   Sell   2    08     100      F  2008
  81   Sell   4    08     100      F  2008
  82    Buy  06    08     100      F  2008
  83   Sell  11    08     100      F  2008
  84    Buy  13    08     100      F  2008
  85   Sell  18    08     100      F  2008
  86    Buy  18    08     100      F  2008
  87   Sell  23    08     100      F  2008
  89   Sell  22    09     100      F  2008
  
  In [3]: df.sort(['Symbol','Year','Month','Day'])
  Out[3]:
     Action Day Month  Shares Symbol  Year
  79   Sell   2    08     100      F  2008
  81   Sell   4    08     100      F  2008
  83   Sell  11    08     100      F  2008
  85   Sell  18    08     100      F  2008
  87   Sell  23    08     100      F  2008
  82    Buy  06    08     100      F  2008
  84    Buy  13    08     100      F  2008
  86    Buy  18    08     100      F  2008
  89   Sell  22    09     100      F  2008
  </code></pre>
  
  <p>Why isn't <a href="http://pandas.pydata.org/pandas-docs/dev/generated/pandas.DataFrame.sort.html" rel="nofollow"><code>sort</code></a> working as expected?</p>