Biophysical Characterization of Complexes of DNA with Mixtures of the Neutral Lipids 1,2-Dioleoyl- sn -glycero-3-phosphoethanolamine- N -hexanoylamine or 1,2-Dioleoyl- sn -glycero-3-phosphoethanolamine- N -dodecanoylamine and 1,2-Dioleoyl- sn -glycero-3-phosphocholine in the Presence of Bivalent Metal Cations for DNA Transfection