title: How to disable button if no checkbox is selected
body: <p>I got the following angular template:</p>
  
  <pre><code>&lt;tr ng-repeat="user in users"&gt;
      &lt;td&gt;&lt;input type="checkbox"&gt;&lt;/td&gt;
      &lt;td&gt;{{user.username}}&lt;/td&gt;
      &lt;td&gt;{{user.apiKey}}&lt;/td&gt;
      &lt;td&gt;{{user.apiSecret}}&lt;/td&gt;
  &lt;/tr&gt;
  ...
  &lt;button type="button" class="btn btn-danger"&gt;&lt;i class="fa fa-trash-o"&gt;&lt;/i&gt;&lt;/button&gt;
  </code></pre>
  
  <p>How can I disable the button if no checkbox is checked but enable it whenever one or more checkboxes in the table are checked?</p>
  
  <p>And how can I determine which table entry was checked and access the user-data?</p>