title: Number of objects with distinct property in a list
body: <p>Here is a simple data set</p>
  
  <pre><code>data = [
      {'artist':'a','song':'a'},
      {'artist':'a','song':'b'},
      {'artist':'a','song':'c'},
      {'artist':'b','song':'d'},
      {'artist':'b','song':'e'},
      {'artist':'c','song':'f'},
      {'artist':'d','song':'g'},
      {'artist':'d','song':'h'},
      {'artist':'d','song':'i'}
  ];
  </code></pre>
  
  <p>Is there any native function in javascript or jQuery that would allow me to get the number of disctinct artist in a concise expression ?</p>