title: groupby and average datetime64
body: <p>how can i have the average date on a groupby </p>
  
  <pre><code>import pandas as pd
  import numpy as np
  df = pd.DataFrame ({'ID' : ['foo', 'bar'] * 5,
                 'VAL' : pd.date_range(start='4/1/2012', periods=10)})
  
  
  ID  VAL
      foo 2012-04-01 00:00:00
      bar 2012-04-02 00:00:00
      bar 2012-04-04 00:00:00
      foo 2012-04-05 00:00:00
      bar 2012-04-06 00:00:00
      foo 2012-04-07 00:00:00
      bar 2012-04-08 00:00:00
      foo 2012-04-09 00:00:00
      bar 2012-04-10 00:00:00
  </code></pre>
  
  <p>desired ouptut :</p>
  
  <pre><code>bar 06/04/2012
  foo 05/04/2012
  </code></pre>
  
  <p>how can i do that ?</p>