Indonesian Medicinal Plants. Part 8. Chemical Structures of Three New Triterpenoids, Bruceajavanin A (Ia), Dihydrobruceajavanin A(II), and Bruceajavanin B (Ib), and a New Alkaloidal Glycoside, Bruceacanthinoside (III), from the Stems of Brucea javanica ( Simaroubaceae)