Fix #114 - ultrasonic sensors

Implements support for the HC-SR04 ultrasonic sensor as an input device
class named DistanceSensor

gpiozero/compat.py

@@ -1,3 +1,5 @@
+# vim: set fileencoding=utf-8:
+
 from __future__ import (
     unicode_literals,
     absolute_import,
@@ -25,3 +27,27 @@ def isclose(a, b, rel_tol=1e-9, abs_tol=0.0):
         (diff <= abs(rel_tol * a)) or
         (diff <= abs_tol)
         )
+
+
+# Backported from py3.4
+def mean(data):
+    if iter(data) is data:
+        data = list(data)
+    n = len(data)
+    if not n:
+        raise ValueError('cannot calculate mean of empty data')
+    return sum(data) / n
+
+
+# Backported from py3.4
+def median(data):
+    data = sorted(data)
+    n = len(data)
+    if not n:
+        raise ValueError('cannot calculate median of empty data')
+    elif n % 2:
+        return data[n // 2]
+    else:
+        i = n // 2
+        return (data[n - 1] + data[n]) / 2
+