make oscar to support django 1.5 AUTH_USER_MODEL feature

Basically i just replace all 'auth.User' with settings.AUTH_USER_MODEL, and put AUTH_USER_MODEL default to 'auth.User' in oscar/defaults.py

all tests passed:
```
    (oscar)ayik:oscar-chenull (issue/472/django1.5)$ ./runtests.py 
    nosetests --verbosity 1 tests -s -x --with-progressive
    Creating test database for alias 'default'...
    
    OK!  520 tests, 0 failures, 0 errors in 20.8s
    Destroying test database for alias 'default'...
    (oscar)ayik:oscar-chenull (issue/472/django1.5)$ 
```
[link to original discussion at mailing list] (https://groups.google.com/d/topic/django-oscar/JJwlin50RYM/discussion)