This work was supported by PEst-C/SAU/UI3282/2011 and PEst2/SAU/LA0001/2011 (FCT, Portugal, and COMPETE). Filipa I. Baptista and Maria J. Pinto acknowledge fellowships from Fundac¸a˜o para a Cieˆncia e a Tecnologia, Portugal (SFRH/BD/35961/2007 and SFRH/BD/51196/2010, respectively). Ramiro D. Almeida is supported by FCT and COMPETE (PTDC/SAU-NEU/104100/2008) and by Marie Curie Actions, 7th Famework programme