Shut down GPIO threads nicely

The motion sensor queue doesn't shut down properly at script end at the
moment and prevents the interpreter shutting down. This is because it's
a non-daemon thread so `__del__` never gets run and so on.

This is a bit of a major PR - I can split it up if you want. Firstly it
makes a common base class called `GPIODevice` for both `InputDevice` and
`OutputDevice`. This just takes care of the read-only pin stuff. Next it
makes a `GPIOThread` class that ensures its a daemon thread, and which
also ensures proper cleanup on shutdown.

Finally, it fixes `MotionSensor` to use the new `GPIOThread` class
(tested this time! Works nicely) and adds the `blink` method to the
`LED` class (which also works nicely this time).

gpiozero/output_devices.py

@@ -1,9 +1,15 @@
 from RPi import GPIO
 
+from .devices import GPIODeviceError, GPIODevice, GPIOThread
 
-class OutputDevice(object):
-    def __init__(self, pin):
-        self.pin = pin
+
+class OutputDeviceError(GPIODeviceError):
+    pass
+
+
+class OutputDevice(GPIODevice):
+    def __init__(self, pin=None):
+        super(OutputDevice, self).__init__(pin)
         GPIO.setup(pin, GPIO.OUT)
 
     def on(self):
@@ -14,7 +20,36 @@ class OutputDevice(object):
 
 
 class LED(OutputDevice):
-    pass
+    def __init__(self, pin=None):
+        super(LED, self).__init__(pin)
+        self._blink_thread = None
+
+    def blink(self, on_time, off_time):
+        self._stop_blink()
+        self._blink_thread = GPIOThread(target=self._blink_led, args=(on_time, off_time))
+        self._blink_thread.start()
+
+    def _stop_blink(self):
+        if self._blink_thread:
+            self._blink_thread.stop()
+            self._blink_thread = None
+
+    def _blink_led(self, on_time, off_time):
+        while True:
+            super(LED, self).on()
+            if self._blink_thread.stopping.wait(on_time):
+                break
+            super(LED, self).off()
+            if self._blink_thread.stopping.wait(off_time):
+                break
+
+    def on(self):
+        self._stop_blink()
+        super(LED, self).on()
+
+    def off(self):
+        self._stop_blink()
+        super(LED, self).off()
 
 
 class Buzzer(OutputDevice):