code: function ajax(opts, cb) {
              opts = opts || {};
              var xmlhttp;
  
              if (typeof window === 'undefined') {
                  // TODO: refactor node usage to live in here
                  return cb();
              }
  
              if (window.XMLHttpRequest) {
                  // code for IE7+, Firefox, Chrome, Opera, Safari
                  xmlhttp = new XMLHttpRequest();
              } else {
                  // code for IE6, IE5
                  xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
              }
  
              xmlhttp.onreadystatechange = function() {
                  if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                      cb(xmlhttp.responseText);
                  }
              };
  
              xmlhttp.open(opts.method, opts.url, true);
              xmlhttp.send();
              return xmlhttp;
          }
docs: Send an AJAX request.
  
  @param  {Object}   opts [optional]
  @param  {Function} cb
  @return {XMLHttpRequest}