Apscheduler

- 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

- Fixed issue (Fixes [18](https://github.com/jcass77/django-apscheduler/issues/18)).
- Added check whether `connections.connection` is `None`.

- This release closes bugs described in (Fixes [15](https://github.com/jcass77/django-apscheduler/issues/15)).
- After updating, please run `./manage.py migrate django_apscheduler` to apply the latest database changes.

- Fix [13](https://github.com/jcass77/django-apscheduler/issues/13).
- Added exception handling when system tables doesn't exists