**12. Acknowledgments**

This work received financial support from the Portuguese State through "Fundação para a Ciência e Tecnologia" (FCT) (project PPCDT/SAU-OBS/55849/2004 & POCI/SAU-OBS/55849/2004). Vera M. Costa acknowledges FCT for her Pos-Doc grant (SFRH/BPD/63746/2009). The authors acknowledge Joana Macedo for her technical assistance with figure 3 included in the manuscript.
