By Ruairidh Villar, Sophie Knight and Brett Wolf TOKYO/ST LOUIS Tue Feb 25, 2014 6:47pm EST (Reuters) – Mt. Gox, once the world’s biggest bitcoin exchange, abruptly stopped trading on Tuesday and its chief executive said the business was at “a turning point,” sparking concerns about the future of the unregulated virtual currency. Several other digital currency exchanges and prominent early-stage investors in bitcoin responded with forceful statements in an attempt to reassure investors of […]