1. Laboratory of Physics and Helsinki Institute of Physics, Helsinki University of Technology, P.O. Box 1100, FI-02015 HUT, Finland, Institute of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland, Wihuri Research Institute, Kalliolinnantie 4, FI-00140 Helsinki, Finland, and MemphysCenter for Biomembrane Physics, Physics Department, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark