1. School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
2. Instituto de Astrofisica de Canarias, Calle Via Lactea s/n, E-38205 La Laguna, Tenerife, Spain
3. Departamento de Astrofisica, Universidad de La Laguna, E-38206 La Laguna, Spain
4. Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
5. Statistics Section, Department of Mathematics, Imperial College London, London SW7 2AZ, UK
6. Department of Astronomy, Stockholm University, Albanova, SE-10691 Stockholm, Sweden