Fix gpiozero#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 :)

Author: waveform80

docs/api_output.rst

@@ -43,6 +43,20 @@ Motor
 .. autoclass:: Motor(forward, backward, pwm=True)
     :members: forward, backward, stop
 
+Servo
+=====
+
+.. autoclass:: Servo(pin, initial_value=0, min_pulse_width=1/1000, max_pulse_width=2/1000, frame_width=20/1000)
+    :inherited-members:
+    :members:
+
+AngularServo
+============
+
+.. autoclass:: AngularServo(pin, initial_angle=0, min_angle=-90, max_angle=90, min_pulse_width=1/1000, max_pulse_width=2/1000, frame_width=20/1000)
+    :inherited-members:
+    :members:
+
 Base Classes
 ============
 

docs/images/composite_device_hierarchy.dot

@@ -32,5 +32,7 @@ digraph classes {
     RyanteckRobot->Robot;
     CamJamKitRobot->Robot;
     Motor->CompositeDevice;
+    Servo->CompositeDevice;
+    AngularServo->Servo;
     Energenie->Device;
 }

docs/images/composite_device_hierarchy.pdf

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