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/__init__.py

@@ -4,14 +4,11 @@ import atexit
 
 from RPi import GPIO
 
-
-def gpiozero_shutdown():
-    GPIO.cleanup()
-
-atexit.register(gpiozero_shutdown)
-GPIO.setmode(GPIO.BCM)
-GPIO.setwarnings(False)
-
+from .devices import (
+    _gpio_threads_shutdown,
+    GPIODeviceError,
+    GPIODevice,
+    )
 from .input_devices import (
     InputDeviceError,
     InputDevice,
@@ -26,3 +23,13 @@ from .output_devices import (
     Buzzer,
     Motor,
 )
+
+
+def gpiozero_shutdown():
+    _gpio_threads_shutdown()
+    GPIO.cleanup()
+
+atexit.register(gpiozero_shutdown)
+GPIO.setmode(GPIO.BCM)
+GPIO.setwarnings(False)
+