Synergistic Effects of Gold Nanoparticles Mixed with Gentamicin, Erythromycin, Clindamycin, Bacitracin, and Polymyxin B against Staphylococcus aureus, Staphylococcus saprophyticus, Staphylococcus epidermidis, Enterococcus faecium and Enterococcus faecalis