As described in issue #91, on Edison setting 0% duty doesn't disable the PWM on a pin completely. Therefore we add a couple of Edison-specific _pre functions and an internal PWM state variable, which we use to toggle PWM enabled/disabled based on what duty is set for the pin. Closes #91. Signed-off-by: Alex Tereschenko <alext.mkrs@gmail.com> Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>