Dynamically add NavigationItem-s
 - extended the DiscountListFragment with NavigationItem and implement
the methods (mainly handle position and name, position is the position
it will get in the navigationItems items list (located in
NavigationManager, used to handle clicks), and the name which is shown
in the menu list
 - DiscountListFragment added a name through the constructor  
 - in the MainActivity added DiscountListFragment to the
NavigationDrawer