Updated User object for Django 1.5 compatibility.

Setting the pk attribute of the User object for Django 1.5 compatibility.

In Django 1.4, the id of the User object was used as the unique identifier. In Django 1.5, the pk is now used instead of id.



Here is the related code change in Django: https://github.com/django/django/commit/0eeae15056edf07f786d3be5b47c14ab62eacd31
Merge pull request #31 from linhuacheng/patch-7

Set the pk of User object for Django 1.5 compatibility.