Acknowledgements
MAA gratefully acknowledges supporting grants from Sweden (VR Dnr 621-2006-3288) and Poland (UMO-2011/01/B/ST9/05439). MAA also thanks the College of Charleston for hosting him during a portion of his work on this review. PCF gratefully acknowledges supporting grants from the National Science Foundation under Grant No. NSF PHY11-25915, the College of Charleston, and the South Carolina Space Grant Consortium. PCF also enjoyed the hospitality of NORDITA and Göteborg University while working on this review. Computational support was provided under the following NSF programs: Partnerships for Advanced Computational Infrastructure, Distributed Terascale Facility (DTF) and Terascale Extensions: Enhancements to the Extensible Terascale Facility.