Refactored sortable_by into a property instead of a classmethod.

Added backwards compatibility for pre 1.1.1 versions that still have sortable_by defined as a classmethod.
Need to address dynamic regroup template tag to see why categories are not being grouped properly in sample app.

adminsortable/templates/adminsortable/change_list.html

@@ -55,7 +55,7 @@
 		{% if objects %}
 		<div id="sortable">
 			{% if group_expression %}
-                            {% render_nested_sortable_objects objects group_expression %}
+                 {% render_nested_sortable_objects objects group_expression %}
 			{% else %}
 			     {% render_sortable_objects objects %}
 			{% endif %}