The first author's research was supported in part by NSERC grant R0370A01, and by a Distinguished Research Professorship at the University of Western Ontario. He was also supported by the Mathematical Sciences Research Institute, Berkeley.
The third author's research was supported in part by NSF grant DMS-0600122.
Department of Mathematics, Middlesex College, University of Western Ontario, London, Ontario N6A 5B7, CANADA
Department of Mathematics, University of Illinois
at Urbana-Champaign, 273 Altgeld Hall, MC-382, 1409 W. Green Street, Urbana, IL, 61801, USA
Department of Mathematics, Davidson College, Box 7046, Davidson, North Carolina 28035-7046, USA