title: Pandas, groupby and summing over specific months
body: <p>I have a DataFrame :</p>
  
  <pre><code>&lt;class 'pandas.core.frame.DataFrame'&gt;
  DatetimeIndex: 982 entries, 2009-10-30 00:00:00 to 2012-12-16 00:00:00
  Data columns (total 4 columns):
  rain        981  non-null values
  temp_max    982  non-null values
  temp_min    982  non-null values
  temp        982  non-null values
  dtypes: float64(4)
  </code></pre>
  
  <p>For summing per Year/Month  i use :</p>
  
  <pre><code>mdata = data.groupby([lambda x: x.year, lambda x: x.month]).agg([sum])
  </code></pre>
  
  <p>But i need Seasonal analysis (summer, winter etc), so how i can create the Sum of specific months like [1 ,2 ,3] of each year?  </p>
  
  <p>Ty </p>