title: Required on a ngTagsInput doesn't work
body: <p>I am using the <a href="http://mbenford.github.io/ngTagsInput/" rel="nofollow">ngTagsInput</a> library in my app. Setting the required keyword is not affecting form validation. Why may this be the case?</p>
  
  <pre><code>&lt;tags-input name="tags" min-length="2" add-on-paste="true" class="bootstrap" ng-model="name" id="name" placeholder="Name" required&gt;&lt;/tags-input&gt;
  </code></pre>