title: ngJsTree Checkbox is not visible
body: <p>I use ngJSTree first time.
  I cannot see checkboxes. In Browser I see style as follows when examine html element:</p>
  
  <pre><code>&lt;a class="jstree-anchor jstree-clicked" href="#" tabindex="-1" id="21_anchor"&gt;
  &lt;i class="jstree-icon jstree-checkbox" role="presentation"&gt;&lt;/i&gt;...
  </code></pre>
  
  <p>Controller uses checkbox plugin:</p>
  
  <pre><code>plugins : ['types','themes', 'checkbox'],
  checkbox : {
                      "override_ui": true,
                      "real_checkboxes": true,
                      "keep_selected_style" : false,
                      "visible": true
                  }
  </code></pre>
  
  <p>I use css from jstree.dist.themes.default.
  JsTree should be easy to use.
  Do I miss some part of configuration?</p>