Fix upload_to_mugshot attr calls on user instance

Code in upload_to_mugshot is calling a user
property on an instance of a user causing an
error and failing unit test.


ERROR: test_upload_mugshot (userena.tests.models.UserenaSignupModelTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/michael/Projects/opensource/django-userena/four/lib/python2.7/site-packages/django_userena-1.1.2-py2.7.egg/userena/models.py", line 34, in upload_to_mugshot
    'username': instance.user.username,
AttributeError: 'User' object has no attribute 'user'