title: Delete associative table rows when deleting a main model entry
body: <p>I have 2 models, usergroups and usergrouppermissions. my usergrouppermissions has a field called user_group_id which links back to the main usergroup. In a case where I want to delete the user group, I automatically want it to delete the entry in the database for usergrouppermissions (as the user group wont exist) anymore. I have tried delete, however, it just deletes the usergroup.</p>
  
  <pre><code>  // Here is my model: (usergroup):
    public $hasMany = array(
         'UserGroupPermission'
    );
  
    // Here is the usergrouppermissions model:
    public $belongsTo = array(
          "UserGroup"
      );
  </code></pre>