Django-apscheduler

- Added timezone support when rendering datetimes; dropped support for django1.8 (
Fixes [43](https://github.com/jcass77/django-apscheduler/issues/43) - thanks jcass77).
- Added model manager for deleting old job executions (
Fixes [58](https://github.com/jcass77/django-apscheduler/issues/58) - thanks jcass77).

- Fixed exception when removing failed jobs (Fixes [33](https://github.com/jcass77/django-apscheduler/issues/33)).
- Accounted for `dt` coming in as `None` into `serialize_dt` (
Fixes [35](https://github.com/jcass77/django-apscheduler/issues/35)).

- Fix of [26](https://github.com/jcass77/django-apscheduler/issues/26).

- This release covers this PR [23](https://github.com/jcass77/django-apscheduler/issues/23), thanks to nialllo

- Now `add_job` with duplicated job `id` will refresh job in database instead of raising an exception.

- Fixed bug [20](https://github.com/jcass77/django-apscheduler/issues/20).
- Changed logger from `default` to `django_apscheduler`.
- Added `on_error_value` into `ignore_database_error` (to return empty array instead of `None` in some methods).
- Added django==1.8 test env in tox.ini