**Acknowledgements**

We are grateful to the Brazilian funding agencies CNPq, FAPESP and CAPES, for their financial support. A.T. Costa, R. K. Makiyama and A. V. Nunes are recipients of doctoral fellowships from CNPq (140299/2010-6), FAPESP (2010/03001-0), CAPES, respectively, and J.P. Bravo is the recipient of a CNPq/PNPD postdoctoral fellowship (558413/2008).
