Make AccessMixin.login_url changeable at runtime

`AccessMixin.login_url` is defined at import time according to `settings.LOGIN_URL` and this applies for all request. There's no way to change it. 

Sometimes it's neccessary to be able to change settings on the fly; for example during testing. Django offers a way to override settings via `django.test.utils.override_settings` but changing `settings.LOGIN_URL` won't affect views that inherit from `AccessMixin` because their `login_url` was set at import time.

This fixes #91.