Due to a lapse the Acknowledgements section was missed from the original article version.
Acknowledgments
The authors would like to thank to Fundação para a Ciência e Tecnologia, Portugal, for financing the Project PTDC/EQU-EQU/104552/2008. Ana Nunes further acknowledges its post-doc grant SFRH / BPD / 74994/ 2010.
© 2012 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).