title: angular ui-grid how to use onRegisterApi in gridOptions
body: <p>I have a editable table with extra rows .columnDefs. I added a grid api to table to get alert when something was change:</p>
  
  <pre><code>$scope.gridOptions.onRegisterApi = function (gridApi) {
          //set gridApi on scope
          $scope.gridApi = gridApi;
          gridApi.edit.on.afterCellEdit($scope, function (rowEntity, colDef, newValue, oldValue) {
              if (newValue !== oldValue) {
                  alert('edited row id:' + rowEntity.id + ' Column:' + colDef.name + ' newValue:' + newValue + ' oldValue:' + oldValue);
              }
          });
      };
  </code></pre>
  
  <p>I don't know how to use it in columnDefs. I tried  <code>$scope.gridOptions.columnDefs.onRegisterApi = function (gridApi) {}</code>
  But it isn't work. I have to get information what was changed in this sub rows.</p>