title: Sum of values with same month
body: <pre><code>data = {'dates': ['2010-01-29', '2011-06-14', '2012-01-18'], 'values': [4, 3, 8]}
  df = pd.DataFrame(data)
  df.set_index('dates')
  df.index = df.index.astype('datetime64[ns]')
  </code></pre>
  
  <p>Having a dataframe where the index is a date, how would I go about adding a new column caled 'Month' that is the sum of all values for that month but doesn't "enter the future" as in saying that it only adds up days before its date. </p>
  
  <p>This is what the column would look like.</p>
  
  <pre><code>'Month': [4, 3, 12] 
  </code></pre>