title: Cumulative sum of values by month, filling in for missing months
body: <p>I have this data table and I'm wondering if is possible create a query that get a cumulative sum by month considering <strong><em>all months until the current month</em></strong>.</p>
  
  <pre><code>date_added                    | qty
  ------------------------------------
  2015-08-04 22:28:24.633784-03 | 1
  2015-05-20 20:22:29.458541-03 | 1
  2015-04-08 14:16:09.844229-03 | 1
  2015-04-07 23:10:42.325081-03 | 1
  2015-07-06 18:50:30.164932-03 | 1
  2015-08-22 15:01:54.03697-03  | 1
  2015-08-06 18:25:07.57763-03  | 1
  2015-04-07 23:12:20.850783-03 | 1
  2015-07-23 17:45:29.456034-03 | 1
  2015-04-28 20:12:48.110922-03 | 1
  2015-04-28 13:26:04.770365-03 | 1
  2015-05-19 13:30:08.186289-03 | 1
  2015-08-06 18:26:46.448608-03 | 1
  2015-08-27 16:43:06.561005-03 | 1
  2015-08-07 12:15:29.242067-03 | 1
  </code></pre>
  
  <p>I need a result like that:</p>
  
  <pre><code>Jan|0
  Feb|0
  Mar|0
  Apr|5
  May|7
  Jun|7
  Jul|9
  Aug|15
  </code></pre>