title: How do I get a Date from a week number?
body: <p>I want to find all items created in a given week, and pass in a week number param. (created_at is a normal timestamp.)</p>
  
  <p>Given a week number, what is the easiest way to find a date in that particular week? 
  (Any date in the week will do, as I will use beginning_of_week and end_of_week in the scope.)</p>