Magneto-Structural Effects of the Jahn-Teller Distortions on 2,2'-Bipyrimidine (bpm)-Bridged Dinuclear Copper(II) Complexes: Crystal Structures and Magnetic Properties of [Cu2(bpm)(H2O)4(SO4)2].3H2O and [Cu2(bpm)(H2O)8](SO4)2.2H2O