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The authors want to thank Daniel Gómez Vergel and the anonymous referee for many interesting comments. This work has been supported by the Spanish MICINN research grants FIS2008-03221, FIS2009-11893 and the Consolider-Ingenio 2010 Program CPAN (CSD2007-00042).
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Living Rev. Relativity 13, (2010), 6
http://www.livingreviews.org/lrr-2010-6 |
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