title: pandas groupby with datetimeindex
body: <pre><code>grouped = data.groupby('LA_DECH')
  start = date(2016, 1, 1)
  end = date(2016, 12, 31)
  rng = pd.date_range(start, end, freq='BM')
  </code></pre>
  
  <p>Is there an easy way to extract data (list of df) with this comparison:
   <code>'2016/1/1' &lt; grouped['LA_DECH] &lt; '2016/2/29'</code> and this for each period in <code>rng</code>?</p>