title: async.js each get index in iterator
body: <p>I'm using <a href="https://github.com/caolan/async/blob/master/README.md" rel="noreferrer">caolan's async.js</a> library, specifically the .each method.</p>
  
  <p>How do you get access to the index in the iterator?</p>
  
  <pre><code>async.each(ary, function(element, callback){
    //do stuff here for each element in ary
    //how do I get access to the index?
  }, function(err) {
    //final callback here
  })
  </code></pre>