This work was supported by the European Commission Seventh Framework Programme under grant agreement no. NMP4-SL-2008-214340, project RESOLVE, KU Leuven (GOA), Funding for Scientific Research—Flanders (F.W.O.), IWT, the Belgian Federal Science Policy Office through IAP 7/05, the Belgian National Fund for Scientific Research (FNRS-FRFC), the OPTI2MAT “Pole d’Excellence” Programme of Region Wallonne, the Spanish MINECO (Project CTQ2010-16339), and DGR, Catalonia (Project 2009 SGR 158). W.J.S. thanks the CSIC for a JAE predoctoral grant co-financed by the FSE. D.B. is FNRS research director. We thank Meir Lahav for useful comments.