Django-hijack

- Make hijacking via POST a default requirement
- Introduce a setting `HIJACK_ALLOW_GET_REQUESTS`
- Move Django admin integration to a separate package as it depends on GET (https://github.com/arteria/django-hijack-admin)
- **If you currently use the admin integration, follow the instructions there when upgrading**
- The setting `HIJACK_DISPLAY_ADMIN_BUTTON` is not used anymore.
- Pass request to signals
- Tests for custom decorators

- Fix end-of-hijack session cleanup code
- Don't save hijacked user at login; disconnect update_last_login instead
- Avoid encoding error in setup.py

- Optional setting for admin hijack button template, cf. http://django-hijack.readthedocs.org/en/latest/configuration/#hijack_button_template
- Minor enhancements

- Use pk to access the User primary key

- Use pk to access the User primary key

- Remove `django-sessioninfo` as a dependency