Fix for php < 5.3 and function name as callback

For actions like this:

`
$bulk_actions->register_bulk_action(array(
				'menu_text' => 'Export',
				'admin_notice' => 'Exported',
				'callback' => 'WPDashboardPostsExporter_AdminController::exportAllToXLS'
			));
`
Merge pull request #1 from antongorodezkiy/patch-1

Fix for php < 5.3 and function name as callback