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/mixins.py

@@ -438,23 +438,28 @@ class HoldThread(GPIOThread):
     device is active.
     """
     def __init__(self, parent):
-        super(HoldThread, self).__init__(target=self.held, args=(parent,))
+        super(HoldThread, self).__init__(
+            target=self.held, args=(weakref.proxy(parent),))
         self.holding = Event()
         self.start()
 
     def held(self, parent):
-        while not self.stopping.is_set():
-            if self.holding.wait(0.1):
-                self.holding.clear()
-                while not (
-                        self.stopping.is_set() or
-                        parent._inactive_event.wait(parent.hold_time)
-                        ):
-                    if parent._held_from is None:
-                        parent._held_from = time()
-                    parent._fire_held()
-                    if not parent.hold_repeat:
-                        break
+        try:
+            while not self.stopping.is_set():
+                if self.holding.wait(0.1):
+                    self.holding.clear()
+                    while not (
+                            self.stopping.is_set() or
+                            parent._inactive_event.wait(parent.hold_time)
+                            ):
+                        if parent._held_from is None:
+                            parent._held_from = time()
+                        parent._fire_held()
+                        if not parent.hold_repeat:
+                            break
+        except ReferenceError:
+            # Parent is dead; time to die!
+            pass
 
 
 class GPIOQueue(GPIOThread):