title: Invisible google Recaptcha and ajax form
body: <p>I have an ajax form:</p>
  
  <pre><code>  &lt;form id="my_form"&gt;
      &lt;input type="text" id="field1" /&gt;
      &lt;input type="submit" value="submit" /&gt;
    &lt;/form&gt;
  </code></pre>
  
  <p>And js code:</p>
  
  <pre><code>document.getElementById("my_form").onsubmit = function(e) {
    e.preventDefault();
  
    var xhr = new XMLHttpRequest();
    //.............. send request to a server
  </code></pre>
  
  <p>In the documentation it assumes that a form is a normal form, not ajax. How exactly should I integrate invisible reCaptcha to my ajax forms? For example:</p>
  
  <pre><code>  &lt;form id="my_form"&gt;
      &lt;input type="text" id="field1" /&gt;
      &lt;div class="g-recaptcha" data-sitekey="12345" data-callback="????&gt;&lt;/div&gt;
      &lt;input type="submit" value="submit" /&gt;
    &lt;/form&gt;
  </code></pre>
  
  <p>And, in particular, what should I specify for "data-callback" handler? Again, in the documentation it  data-callback submits a form, but a normal form, whereas mine is ajax. Do I need "data-callback" at all? Shouldn't I instead call recaptcha inside my handler? How?</p>
  
  <p>There're "render", "getResponse" and "execute". Which one should I use? It's not clear from the documentation.</p>