Fix #248

Add Servo and AngularServo implementation along with docs and tests.
This is a deliberately minimal implementation designed to be added to as
we agree on new extensions (better than making an all-singing,
all-dancing version in which I get things wrong and then wind up making
backward incompatible changes to get it right :)

gpiozero/__init__.py

@@ -100,6 +100,8 @@ from .output_devices import (
     LED,
     Buzzer,
     Motor,
+    Servo,
+    AngularServo,
     RGBLED,
 )
 from .boards import (