1. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
2. Kavli Institute of Cosmology Cambridge, Madingley Road, Cambridge CB3 0HA, UK
3. National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA
4. Physics Department, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA
5. SKA Organization, Lower Withington Macclesfield, Cheshire SK11 9DL, UK
6. Battcock Centre for Experimental Astrophysics, Cavendish Laboratory, Cambridge CB3 0HE, UK
7. Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
8. Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
9. Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany