Acknowledgements
The authors acknowledge the support of the Centre for Development Cooperation, Universitat Politècnica de Catalunya e BarcelonaTech (Project CCD2010-O031). Marianna Garfí is grateful to the Secretary General of Universities, Ministry of Education (Spain)(Programa Nacional de Movilidad de Recursos Humanos del Plan Nacional de I-Dþi 2008e2011). Erasmo Cadena is grateful to the CONACyT (México) for the award of a Postdoctoral fellowship.