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/mixins.py

@@ -70,10 +70,13 @@ class SourceMixin(object):
 
     def close(self):
         try:
-            super(SourceMixin, self).close()
             self.source = None
         except AttributeError:
             pass
+        try:
+            super(SourceMixin, self).close()
+        except AttributeError:
+            pass
 
     def _copy_values(self, source):
         for v in source: