Fix invalid state, which happen if signals raise exception

If "super(ThreadedComment, self).save(*args, **kwargs)"  is not last in save method,
then exceptions in functions called by post_save signal will lead to invalid DataBase state.

For example: field `tree_path` in DB will be ""(empty string), then
   def root_id(self):
        return int(self.tree_path.split(PATH_SEPARATOR)[0])
equivalent to  int('') and return
        ValueError: invalid literal for int() with base 10: ''
so whole page with comments will be broken.




When "super .. .save(*args, **kwargs)" was in the




Tested on Django 1.6, Postgre