sphinx_lesson/exerciselist: Set default includenodes and excludenodes

- Ref: #107
- There was a problem where the includenodes and excludenodes
  attributes aren't set in singlehtml and pdf builds.  I guess it's
  something about the order of initialzation being wrong.  Anyway, in
  existing builds these aren't correct and the exercise list appears
  empty.  This fixes builds and then we can fix things more later.

sphinx_lesson/exerciselist.py

@@ -23,6 +23,8 @@ class exerciselist(nodes.General, nodes.Element):
 
     Gets replaced with contents in the second pass.
     """
+    include_classes = frozenset()
+    exclude_classes = frozenset()
     def __init__(self, *args,
                  include_classes=DEFAULT_EXERCISELIST_CLASSES,
                  exclude_classes=DEFAULT_EXERCISELIST_CLASSES_EXCLUDE):