The antimicrobial activity of the methanolic extract from the stem bark of Irvingia gabonensis (IGM), fractions and compounds isolated from IGM [3-friedelanone (1), betulinic acid (2), oleanolic acid (3), 3,3',4'-tri-O-methylellagic acid (4), 3,4-di-O-methylellagic acid (5) and hardwickiic acid (6)]