[WIP] Django 1.5 support

I started to work on Django 1.5 support. There are several changes we will need to reflect:

* changes to [user class](https://github.com/django/django/commit/70a0de37d132e5f1514fb939875f69649f103124). This isn't very tricky, you just have to use simple try-raise block, as presented in my first commit. The only problem I encountered so far are migrations, which are referencing `auth.User` directly. I'm not really sure how to fix it or if it's even possible. For now I deleted the directory in my local repo.
* patch requests in test client seems to be borked because of [this commit](https://github.com/django/django/commit/d7dfab59ead97b35c6f6786784225f377783e376). I *think* only test client is affected though. Will have to look bit more into it.
* it might be a good idea to refactor patch and put methods to support [`update_fields`](https://docs.djangoproject.com/en/dev/releases/1.5/#support-for-saving-a-subset-of-model-s-fields) kwarg in save method. I'm not entirely sure how it's implemented in Django, will have to look at it.
* we will need new unit tests for these changes

I'll work on it a bit more once I have more time.