remove db hit in nav_menu_item_parent_classing()

refactor nav_menu_item_parent_classing() to use property on parameter
$item in place of performing a database call to determine if a menu item
needs the class "has-dropdown"
Merge pull request #48 from salcode/nav-item-parent-class-remove-db-call

remove db hit in nav_menu_item_parent_classing()