Django-db-mailer

* Drop Django <1.8 support
* Add Django 2.1 support
* Make app more robust without celery installed (107)

* Migration fixes
* Providers compatibility with Python 3.x
* Docs update

- Telegram backend
- Django 1.10 compatibility fixes

- Django 1.10 support
- Default mail provider
- WebPush docs and examples (GC/FF/Safari)
- Some fixes on push notification providers
- Push subscriptions handlers
- Add smsbliss provider
- APNs/2: fix utf8
- Push backend, push provider, model data and title fields
- Logging fix