title: Ruby group by month and year
body: <p>I have an (ar) class PressClipping consisting of a title and a date field.</p>
  
  <p>I have to display it like that:</p>
  
  <p>2011 February<br>
    title1<br>
    title2<br>
    ...<br>
  2011 January<br>
  ...  </p>
  
  <p>What is the easiest way to perform this grouping?</p>