TLB would like to acknowledge support by the NSF and AFOSR for this research. In addition, he thanks Michael Merrick and Karthik Iyer for their contributions and Daan Frenkel and the FOM Institute in Amsterdam for their hospitality and many informative discussions during a sabbatical leave there. RDC wishes to acknowledge the important contributions of his collaborators Anthony Duncan, Sanford Asher, Alan Walsh and Nir Ben-Tal. He is also grateful for support from the NSF, AFOSR and ONR (grant N00014-94-0592).