Add an adapter so Django stores the right value

[bhcarpenter]

Add an adapter so Django stores the right value into session. We want to store and use the user_id property instead of the id.

Retrieving the User via user_id was already taken care of. This takes care of storing the user_id.

Refs #152

[mhluongo]

I'm a little concerned because of some edge cases like querying- eg User.objects.get(id=5). The resulting object will have act like this

>>> user = User.objects.get(id=5)
>>> user.id
5
>>> user.user_id
# some other id
>>> user.id == user.user_id
False

OTOH,

>>> user = login(...)
>>> user.id
# some other id
>>> user.id == user.user_id
True

Obviously this particular example is easy to test so I'll get on that.

I'm going to leave this open and see if I can get primary_key=True working properly (and really just explore further)- if there's still a problem we might have to do this or something similar.

[bhcarpenter]

Yep that's exactly right (and even intended).

Unfortunately, django.contrib.auth persists the id field of the user given to login, so if we want to have get_user retrieve by user_id instead later, we'll have to fake it somehow.

I agree that this is probably going to cause issues (and I already know that it breaks django_social_auth). Let's keep talking offline.