title: pd.to_datetime change date format producing wrong dates
body: <p>I have extracted the table below from a csv file :</p>
  
  <p>timestamp &nbsp;    user_id &nbsp;  cost &nbsp; val1<br/>
  01/01/2011 &nbsp; &nbsp;        1 &nbsp; &nbsp;     1 &nbsp; &nbsp; 3<br/>
  01/07/2012 &nbsp; &nbsp;        1 &nbsp; &nbsp;     19 &nbsp; &nbsp;        57<br/>
  01/09/2013 &nbsp; &nbsp;        1 &nbsp; &nbsp;     21 &nbsp; &nbsp;        63<br/>
  01/02/2011 &nbsp; &nbsp;        2 &nbsp; &nbsp;     20 &nbsp; &nbsp;        8<br/></p>
  
  <p>And I for this purpose, I used the following statements :
  import pandas as pd</p>
  
  <pre><code>newnames = ['date','user_id', 'cost', 'val1']
  df = pd.read_csv('expenses.csv', names = newnames, header = False)
  df['timestamp'] = pd.to_datetime(df['timestamp'],format='%d%m%Y')
  </code></pre>
  
  <p>But the dates of table obtained in df are wrong : months and days are permutated without reason, i.e 01/09/2013 becomes 09/01/2013 without reason.</p>
  
  <p>Many thanks in advance for your help.</p>