title: How to resample pandas series monthy (not end, not start)?
body: <p>I know how to resample a dataframe but feel frustrated by rule parameter not accepting DateOffset(months=1).</p>
  
  <p>How can I resample with monthly frequency but from any give date. For example:</p>
  
  <pre><code>    ts = pd.date_range(end='19/9/2015', periods=127, freq='D')
      df = pd.DataFrame(pd.Series(range(len(ts)), index=ts))
      print(df.resample('M',how='sum',convention='end').tail(3))
  </code></pre>
  
  <p>will give me </p>
  
  <pre><code>               0
  2015-07-31  1674
  2015-08-31  2635
  2015-09-30  2090
  </code></pre>
  
  <p>where I wanted it to give</p>
  
  <pre><code>               0
  2015-07-19  ...
  2015-08-19  ...
  2015-09-19  ...
  </code></pre>
  
  <p>How can I achieve this ?</p>