added support for .notation in ArrayObjectAccessor

now you are able to define column like this
$grid->addColumnText('firstName', 'Name')
                ->setSortable()
                ->setColumn('contact.firstName')
                ->setFilterText();

instead of

$renderer = function($row) {
  return $row->contact->firstName;
};
$grid->addColumnText('firstName', 'Name')
                ->setSortable()
                ->setColumn('contact.firstName')
                ->setCustomRender($renderer)
                ->setCustomRenderExport($renderer)
                ->setFilterText()
                    ->setSuggestion($renderer);