title: pandas or numpy to mean the sum of three months. Return as list
body: <pre><code>Date
  2013-11-30    30
  2013-12-31    40
  2014-01-31    50
  2014-02-28    10
  2014-03-31    20
  2014-04-30    30
  2014-05-31    80
  2014-06-30    90
  2014-07-31    10
  
  df = pandas.Dataframe()
  </code></pre>
  
  <p>Lets say I have a <code>Dataframe</code> as above. How can I make a mean of the months for every third month. And return it as a list. </p>
  
  <pre><code>lst[0] = (30 + 40 + 50)/3
  lst[1] = (10 + 20 + 30)/3
  lst[2] = (90 + 90 + 10)/3
  </code></pre>
  
  <p>Are there any smart ways of achieving this with <code>pandas</code> or <code>numpy</code>, as I'd like to keep the amount of days to the month, in case the <code>dataframe</code> could consist of days instead.</p>