Django-compat

* Django 2.0 compatibility

* Added Support for Django 1.11
* Reworked tox and travis environments

- Update existing patches for Django 1.10

- Add `get_current_site` and `admin_utils`

- Backport new `render_to_string` function signature to Django < 1.8
- Backport `verbatim` tag to Django 1.4
- Add `get_template_loaders`
- Add `close_connection`
- Improve `JsonResponse` backport to Django 1.4
- Add tests for `import_module`, `get_model` and `add_to_builtins`
- Anticipate renaming of django.core.urlresolvers to django.urls in 1.10
- Avoid warnings in setup.py

- Python <= 3.x compatibility