This work was supported by the Ministerio de Ciencia e Innovación of Spain (Project CTQ 2007-61126). G.de G. and V.G.F. thank MICINN for personal grants (Juan de la Cierva and Ramon y Cajal program, respectively). I.L. thanks the Principado de Asturias for personal funding (Clarín Program). The authors thank the Brazilian and Spanish agencies CNPq, FUNCAP, PRONEX, CAPES-DGU (Process:149/07) for fellowships and financial suppor
