Semi-generic registration mechanism for modules. Still need to clean up references in content_parser

--HG--
branch : pmitros-mod-template

courseware/modules/x_module.py

@@ -10,7 +10,8 @@ class XModule(object):
     '''
     id_attribute='name' # An attribute guaranteed to be unique
 
-    def get_xml_tags():
+    @classmethod
+    def get_xml_tags(c):
         ''' Tags in the courseware file guaranteed to correspond to the module '''
         return []