title: Pandas dataframe group by multiple columns
body: <p>Given a dataframe with two datetime columns <code>A</code> and <code>B</code> and a numeric column <code>C</code>, how to group by <code>month</code> of both <code>A</code> and <code>B</code> and <code>sum(C)</code> i.e. </p>
  
  <pre><code>In [1]: df
  Out[1]: 
  
        A           B            C  
  0  2013-01-01  2013-01-01  0.282863 
  1  2013-01-02  2013-01-01  0.173215 
  2  2013-02-03  2013-02-04  2.104569 
  3  2013-02-09  2013-04-15  0.706771 
  4  2013-03-05  2013-08-01  0.567020 
  5  2013-03-06  2013-04-01  0.113648
  </code></pre>