Return iterables throughout for consistency.

CHANGES.rst

@@ -7,8 +7,8 @@
 * ``Client.query`` now accepts keyword arguments
   and parameters passed directly to Wolfram|Alpha
   as URL parameters.
-* ``Result.pods`` now returns an iterator and not
-  a list.
+* ``Result.pods`` and ``Pod.subpods`` now returns
+  an iterator and not a list.
 * ``Pod`` objects are no longer iterable. To access
   the subpods of a pod, simply use the ``Pod.subpod``
   property.