BF: new Kayborad class was returning negative durations (but correct vals)

We were subtracting start-stop not stop-start!

Author: peircej

psychopy/hardware/keyboard.py

@@ -385,7 +385,7 @@ def _processEvts(self):
             else:
                 for key in self._keysStillDown:
                     if key.code == evt['keycode']:
-                        key.duration = key.tDown - evt['time']
+                        key.duration = evt['time'] - key.tDown
                         self._keysStillDown.remove(key)
                         break  # this key is done
                     else: