1. FOM Institute for Atomic and Molecular Physics, Science Park 113, 1098 XG Amsterdam, The Netherlands, WZI and HIMS, University of Amsterdam, Nieuwe Achtergracht 129, 1018 WS Amsterdam, The Netherlands, and Laboratoire de Physique Statistique, Ecole Normale Superieure, 24 Rue Lhomond, 75231 Paris cedex 05, France