Fix #553

Ensure SourceMixin descendents shut down the source prior to closing.
Furthermore, make sure devices are closed before pin factory shuts down,
and that pins have a strong reference to their owning factory (to
prevent losing the factory before the pins).

gpiozero/exc.py

@@ -145,6 +145,9 @@ class PinNoPins(PinError, RuntimeError):
 class PinInvalidPin(PinError, ValueError):
     "Error raised when an invalid pin specification is provided"
 
+class PinReservationsExist(PinError, RuntimeError):
+    "Error raised when pin factory is changed while reservations still exist"
+
 class GPIOZeroWarning(Warning):
     "Base class for all warnings in GPIO Zero"