Add close method and context manager

Permit devices to be explicitly closed and used as context managers.
Also deal with cleanup properly at script end and ensure objects don't
step on the global cleanup function.

gpiozero/__init__.py

@@ -1,11 +1,6 @@
 from __future__ import absolute_import
 
-import atexit
-
-from RPi import GPIO
-
 from .devices import (
-    _gpio_threads_shutdown,
     GPIODeviceError,
     GPIODevice,
 )
@@ -35,11 +30,3 @@ from .boards import (
     TrafficHat,
 )
 
-
-def gpiozero_shutdown():
-    _gpio_threads_shutdown()
-    GPIO.cleanup()
-
-atexit.register(gpiozero_shutdown)
-GPIO.setmode(GPIO.BCM)
-GPIO.setwarnings(False)