fix recursive prefix in __flatten_patterns_tree

Changed one line in urlparser.__flatten_pattern_tree__ to allow recursion into lower levels which is necessary when DRFs new routers (which appear as URLResolvers including a list instead of a module) are used in an  app level urls.py. In the original version the prefix of the higher level was lost.

Did not add tests yet. Will improve if not merged by 8/16/2013

Falk