title: How to have a checkbox in an ng-grid grouping
body: <p>Is it possible to have a check box in an ng-grid grouping header? 
  When that grouping check box is clicked, all the checkboxes of the rows under that particular group should be selected.
  The other group rows should remain un-selected.</p>
  
  <p>Right now, when I have my gridOptions as follows, I see checkboxes for all rows, and in the table header. When a column header is dragged to use it for grouping, the grouped rows-header do not have checkboxes. 
  Can anybody help?</p>
  
  <pre><code>$scope.gridOptions = {
      data: 'myData',
      showGroupPanel: true,
      showSelectionCheckbox: true,
      jqueryUITheme: true,
      enableCellSelection: true,
      enableRowSelection: true,
      enableCellEdit: false,
      pinSelectionCheckbox: false,
      selectWithCheckboxOnly: true
  };
  </code></pre>