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/devices.py

@@ -0,0 +1,43 @@
+from threading import Thread, Event
+
+from RPi import GPIO
+
+
+class GPIODeviceError(Exception):
+    pass
+
+
+class GPIODevice(object):
+    def __init__(self, pin=None):
+        if pin is None:
+            raise GPIODeviceError('No GPIO pin number given')
+        self._pin = pin
+
+    @property
+    def pin(self):
+        return self._pin
+
+
+_GPIO_THREADS = set()
+def _gpio_threads_shutdown():
+    while _GPIO_THREADS:
+        for t in _GPIO_THREADS.copy():
+            t.stop()
+
+
+class GPIOThread(Thread):
+    def __init__(self, group=None, target=None, name=None, args=(), kwargs={}):
+        super(GPIOThread, self).__init__(group, target, name, args, kwargs)
+        self.stopping = Event()
+        self.daemon = True
+
+    def start(self):
+        self.stopping.clear()
+        _GPIO_THREADS.add(self)
+        super(GPIOThread, self).start()
+
+    def stop(self):
+        self.stopping.set()
+        self.join()
+        _GPIO_THREADS.discard(self)
+