Stock futures lower as countdown to Yellen speech begins

* Investors have been waiting for Fed Chair Janet Yellen's address at an annual meeting of global central bankers in Jackson Hole, Wyoming, for signals on the timing of the next rate hike. * While traders have priced in a marginal chance of a hike when the Fed meets in September, Yellen's language will be assessed to see whether she endorses the recent hawkish stand taken by key policymakers, especially after considerably strong economic data.