Add callback after grid layout.

[antisilent]

re: #115: Added a callback when grid layout has finished...

<!-- template -->
<div angular-grid="items" ag-callback="agCallback()"></div>

// controller
function gridController($scope) {
  $scope.agCallback = function () {
    // emit an event to grid items, for example
    $scope.$broadcast('angulargrid:done');
  }
}