Fix #64

Make MotionSensor more responsive by effectively removing the averaging
queue. Also add note on how to smooth out "jittery" PIR sensors by
increasing the queue length back up to 5.

gpiozero/input_devices.py

@@ -404,9 +404,14 @@ class MotionSensor(SmoothedInputDevice):
     and GND. VCC should be connected to the Pi's +5V pin, GND to one of the
     Pi's ground pins, and finally OUT to the GPIO specified as the value of the
     `pin` parameter in the constructor.
+
+    This class defaults `queue_len` to 1, effectively removing the averaging
+    of the internal queue. If your PIR sensor has a short fall time and is
+    particularly "jittery" you may wish to set this to a higher value (e.g. 5)
+    to mitigate this.
     """
     def __init__(
-            self, pin=None, queue_len=5, sample_rate=10, threshold=0.5,
+            self, pin=None, queue_len=1, sample_rate=10, threshold=0.5,
             partial=False):
         super(MotionSensor, self).__init__(
             pin, pull_up=False, threshold=threshold,