Fix #107

Add forward_device and backward_device to Motor, left_motor and
right_motor to Robot, and ensure all CompositeDevice descendents have a
proper close() method and closed property. Also, add a few more
_check_open calls around the place to make sure GPIODeviceClosed is
properly raised in response to read and writing values.

gpiozero/boards.py

@@ -31,6 +31,10 @@ class LEDBoard(SourceMixin, CompositeDevice):
         for led in self.leds:
             led.close()
 
+    @property
+    def closed(self):
+        return all(led.closed for led in self.leds)
+
     @property
     def leds(self):
         """
@@ -188,6 +192,10 @@ class TrafficLightsBuzzer(SourceMixin, CompositeDevice):
         self.buzzer.close()
         self.button.close()
 
+    @property
+    def closed(self):
+        return all(o.closed for o in self.all)
+
     @property
     def all(self):
         """
@@ -305,6 +313,24 @@ class Robot(SourceMixin, CompositeDevice):
         self._left.close()
         self._right.close()
 
+    @property
+    def closed(self):
+        return self._left.closed and self._right.closed
+
+    @property
+    def left_motor(self):
+        """
+        Returns the `Motor` device representing the robot's left motor.
+        """
+        return self._left
+
+    @property
+    def right_motor(self):
+        """
+        Returns the `Motor` device representing the robot's right motor.
+        """
+        return self._right
+
     @property
     def value(self):
         """