Fix #91

Override toggle() in PWMOutputDevice, and add reverse() to Motor and
Robot

gpiozero/boards.py

@@ -347,6 +347,16 @@ class Robot(SourceMixin, CompositeDevice):
         self._left.forward(speed)
         self._right.backward(speed)
 
+    def reverse(self):
+        """
+        Reverse the robot's current motor directions. If the robot is currently
+        running full speed forward, it will run full speed backward. If the
+        roboto is turning left at half-speed, it will turn right at half-speed.
+        If the robot is currently stopped it will remain stopped.
+        """
+        self._left.value = -self._left.value
+        self._right.value = -self._right.value
+
     def stop(self):
         """
         Stop the robot.