Bridgehead Nitrogen Heterocyclic Systems: Synthesis and Antimicrobial Activity of s-Triazolo[3,4-b][1,3,4]thiadiazoles, s-Triazolo[3,4-b][1,3,4]thiadiazines and s-Triazolo[3′,4′:2,3]thiadiazino[5,6-b]quinoxaline.