title: parsley.js: Error trying to add custom validator
body: <p>I am trying to add a custom validator</p>
  
  <pre><code>window.ParsleyValidator
    .addValidator('invalidWords', {
      requirementType: 'regexp',
      validateString: function(value, requirement) {
          console.log(value,requirement);
        return /^\b(?:Stadium|GT|BB|HB|Simul|VNOSE|LT|combination|LT1|SSGT|BW|HBS|simul|combo|2hbs|4d|lt2|theatre)\b$/i.test(value)
      },
      messages: {
        en: 'Invalid words detected.'
      }
    });
  </code></pre>
  
  <p>with HTML as...</p>
  
  <pre><code>&lt;div class="col-sm-6 col-lg-6"&gt;
      &lt;div class="form-group"&gt;
          &lt;input type="text" name="company" data-parsley-group="eventinfo" id="Company" class="form-control" placeholder="Company" maxlength="60" tabindex="3" title="Company" parsley-trigger="keyup" data-parsley-invalidwords="" value="#request.args.company#"&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>I keep getting the error <code>Callback must be instanciated with a function</code> and I am not sure why. I am following the directions on the site. <a href="http://parsleyjs.org/doc/index.html#custom" rel="nofollow">http://parsleyjs.org/doc/index.html#custom</a>
  It also doesn't seem to trigger on keyup either.
  using version <code>2.0.7</code></p>