title: Aggregate $group for multiple date ranges
body: <p>In my aggregate, each document in the stream will have a date on it.</p>
  <p>I need to sum some values over date ranges..</p>
  <p>My documents look like:</p>
  <pre><code>{ value: 3,  date: [SoME TIME STAMP] },
  { value: 4,  date: [SoME TIME STAMP] },
  { value: 1,  date: [SoME TIME STAMP] },
  { value: -6, date: [SoME TIME STAMP] }
  </code></pre>
  <p>I want to be able to group these document base on the date range. i.e.: <code>1-7 days ago</code>, <code>8-15 days ago</code>. and <code>15-30 days ago</code>.</p>
  <p>I probably apply 3 different aggregate queries with 3 different $match on dates.</p>
  <p>But is it possible to do all the $group and sum the &quot;value&quot; field in one run?</p>