This research was funded by projects PB96-0861 and CGL2009-12767-C02-02 to M.T. F.M. was supported by a doctoral grant contract funded by the project CGL2004-01872/BOS to M.T. C.P. was supported by grants from the Danish Natural Sciences Research Council (21-01-0526 and 21-03-0125) and the Marie Curie Fellowship of the European Community Host Development program under contract nos.HPMD-CT-2000-00009, REN2001-2647/GLO and MMA-86-2003 to A.G.N.