title: Magento : Custom attributes added in the api2.xml (custom module) are not visible in admin
body: <p>I'm currently trying to extend the Magento Rest API.
  I need to add some features like :</p>
  
  <ul>
  <li>Retrieve the categories</li>
  <li>Add a category</li>
  <li>etc.</li>
  </ul>
  
  <p>The problem is that I'm not able to allow a Rest Role to read the new attributes added with the new module.</p>
  
  <p>In "Rest - Roles", there is no problem, I can allow my role to access the "retrieve" function, but when I go to the "Rest - Attributes", nothing appears so it's impossible to allow anything for my new module..</p>
  
  <p>If I try to send the request I have an error : </p>
  
  <ul>
  <li>403 : Forbidden {"messages":{"error":[{"code":403,"message":"Access denied"}]}}.</li>
  </ul>
  
  <p>If somebody can help me.. Thanks !</p>
  
  <p><em>api2.xml</em></p>
  
  <pre><code>&lt;config&gt;
  &lt;api2&gt;
      &lt;resource_groups&gt;
          &lt;categoriesmanagement translate="title" module="api2"&gt;
              &lt;title&gt;ExtendedRest API&lt;/title&gt;
              &lt;sort_order&gt;10&lt;/sort_order&gt;
          &lt;/categoriesmanagement&gt;
      &lt;/resource_groups&gt;
      &lt;resources&gt;
          &lt;categoriesmanagement translate="title" module="api2"&gt;
              &lt;group&gt;categoriesmanagement&lt;/group&gt;
              &lt;model&gt;categoriesmanagement/api2_category&lt;/model&gt;
              &lt;title&gt;Category Management&lt;/title&gt;
              &lt;sort_order&gt;10&lt;/sort_order&gt;
              &lt;privileges&gt;
                  &lt;admin&gt;
                      &lt;retrieve&gt;1&lt;/retrieve&gt;
                  &lt;/admin&gt;
              &lt;/privileges&gt;
              &lt;attributes&gt;
                  &lt;entity_id&gt;Category ID&lt;/entity_id&gt;
                  &lt;name&gt;Name&lt;/name&gt;
                  &lt;parent_id&gt;Category Parent ID&lt;/parent_id&gt;
                  &lt;is_active&gt;Active&lt;/is_active&gt;
                  &lt;level&gt;Level&lt;/level&gt;
                  &lt;position&gt;Position&lt;/position&gt;
                  &lt;children&gt;Children Ids&lt;/children&gt;
                  &lt;url_key&gt;URL key&lt;/url_key&gt;
                  &lt;store_id&gt;Store ID&lt;/store_id&gt;
              &lt;/attributes&gt;
              &lt;routes&gt;
                  &lt;route_collection&gt;
                      &lt;route&gt;/extendedrest/categoriesmanagement&lt;/route&gt;
                      &lt;action_type&gt;collection&lt;/action_type&gt;
                  &lt;/route_collection&gt;
              &lt;/routes&gt;
              &lt;versions&gt;1&lt;/versions&gt;
          &lt;/categoriesmanagement&gt;
      &lt;/resources&gt;
  &lt;/api2&gt;&lt;/config&gt;
  </code></pre>