title: How to use executeAsync in nightwatch.js?
body: <p>I want do some automated testing useing nightwatch:</p>
  
  <pre><code>this.demoTest = function (browser) {
      browser.executeAsync(function(data, done) {
          someAsyncOperation(function() {
          done(true);
      });
      }, [imagedata], function(result) {
          // ...
      });
  };
  </code></pre>
  
  <p>But I don't know how to use executeAsync, flowing is my code:</p>
  
  <pre><code>module.exports = {
      'Demo asynchronous' : function(client){
          client.url(client.launchUrl);
          client.executeAsync(function(data, done) {
              someAsyncOperation(function() {
                  client.setValue('#PoiSearch', data); 
                  client.click('#POISearchButton');
                  done(true);
          });
          }, ['hotle'], function(result) {
              client.expect.element('#Map div[name*="mark"]').to.be.present;
          });
      }
  }
  </code></pre>
  
  <p>I just want to input a word, than click the search button, as a result I want to know the DOM have the special element .but I don't konw how to use executeAsync.</p>