By Frank Jack Daniel and Sanjeev Miglani NEW DELHI, Jan 13 (Reuters) - In a grubby room, one wall lined with legal tomes, a father and his son leaf through thick case files in preparation for the trial of their lives - defending the main accused in a gang-rape that outraged India and caused shock waves around the world. The small New Delhi legal firm, with its headquarters in a cramped office above a local bank, has been thrust into the international spotlight after being appointed to represent bus driver Ram Singh.