Tax Dept starts probe into Bitcoin exchanges to ascertain rate they can be taxed under.   The indirect tax department has launched an investigation into Bitcoin exchanges operating in India to ascertain at what rate they can be taxed under the goods and services tax (GST) regime, two people with direct knowledge of the matter said. The development comes as the income tax department launched searches on top Bitcoin exchanges including Zebpay, Unocoin and CoinSecure on Wednesday. According to the…