Custom user support

Here is my fix for #46.

I've also replaced `auth.models.User` for `auth.get_user_model` in tests. 

But I didn't find a way to run a single test suite with different settings =(

`@override_settings` isn't helpful because I need to change `INSTALLED_APPS` and to recreate the test database.

So I just manually comment/uncoment [custom user related settings.](https://github.com/matklad/django-simple-history/blob/custom_user_support/runtests.py#L27)

What is the best way to solve this? I see two possibilities

* add `--custom-user` flag to runtests.py
* in runtests.py determine the version of django and use custom user if it is >= 1.5.