Square, the mobile payments firm, is running a trial that allows some users of its Cash platform to buy and sell bitcoin. The news sent Square's stock to an all-time high of $41.80 per share soon after the markets first opened. Bitcoin is also in the green today. The cryptocurrency, known for its wild price swings, was trading up more than 10%. Square added the feature to Cash, a rival to Venmo, because users asked for it.