title: r convert data frame to list of lists
body: <p>Given the following data frame:</p>
  
  <pre><code>Date       Person   Article
  ---------------------------
  01-01-2015 Bob      TV
  01-01-2015 Bob      Video
  01-01-2015 Pete     Book
  02-01-2015 Rob      Skate
  02-01-2015 Kate     TV
  </code></pre>
  
  <p>How could I convert this data frame to a list of list (or multidimensional array) so, that</p>
  
  <pre><code>my_ist[[01-01-2015]][[Bob]]
  </code></pre>
  
  <p>would give</p>
  
  <pre><code>TV, Video
  </code></pre>
  
  <p>?</p>