Rework when_changed attribute to use weakrefs

Some fairly major changes to ensure that the Pin.when_changed property
doesn't keep references to the objects owning the callbacks that are
assigned. This is vaguely tricky given that ordinary weakref's can't be
used with bound methods (which are ephemeral), so I've back-ported
weakref.WeakMethod from Py3.4.

This solves a whole pile of things like Button instances not
disappearing when they're deleted, and makes composite devices
containing Buttons much easier to construct as we don't need to worry
about partially constructed things not getting deleted.

gpiozero/pins/rpigpio.py

@@ -7,6 +7,14 @@ from __future__ import (
 str = type('')
 
 import warnings
+from types import MethodType
+from threading import RLock
+from weakref import ref
+try:
+    from weakref import WeakMethod
+except ImportError:
+    from .compat import WeakMethod
+
 from RPi import GPIO
 
 from .local import LocalPiFactory, LocalPiPin
@@ -89,6 +97,7 @@ class RPiGPIOPin(LocalPiPin):
         self._frequency = None
         self._duty_cycle = None
         self._bounce = -666
+        self._when_changed_lock = RLock()
         self._when_changed = None
         self._edges = GPIO.BOTH
         GPIO.setup(self.number, GPIO.IN, self.GPIO_PULL_UPS[self._pull])
@@ -202,19 +211,15 @@ class RPiGPIOPin(LocalPiPin):
         finally:
             self.when_changed = f
 
-    def _get_when_changed(self):
-        return self._when_changed
+    def _call_when_changed(self, channel):
+        super(RPiGPIOPin, self)._call_when_changed()
 
-    def _set_when_changed(self, value):
-        if self._when_changed is None and value is not None:
-            self._when_changed = value
-            GPIO.add_event_detect(
-                self.number, self._edges,
-                callback=lambda channel: self._when_changed(),
-                bouncetime=self._bounce)
-        elif self._when_changed is not None and value is None:
-            GPIO.remove_event_detect(self.number)
-            self._when_changed = None
-        else:
-            self._when_changed = value
+    def _enable_event_detect(self):
+        GPIO.add_event_detect(
+            self.number, self._edges,
+            callback=self._call_when_changed,
+            bouncetime=self._bounce)
+
+    def _disable_event_detect(self):
+        GPIO.remove_event_detect(self.number)