title: Pandas - cumsum by month?
body: <p>I have a dataframe that looks like this:</p>
  
  <pre><code>Date          n
  2014-02-27    4
  2014-02-28    5
  2014-03-01    1
  2014-03-02    6
  2014-03-03    7
  </code></pre>
  
  <p>I'm trying to get to one that looks like this</p>
  
  <pre><code>Date          n    csn
  2014-02-27    4    4
  2014-02-28    5    9
  2014-03-01    1    1
  2014-03-02    6    7
  2014-03-03    7    14
  </code></pre>
  
  <p>...i.e. I want a column with the running total within the month and I want it to start over each month. How can I do this?</p>