Change CronJobManager from settings and control day of run times in schedule

[peynaj]

Summary of changes (document):

1. Access to change CronJobManager from django settings if required.
2. Add day config to CronJob schedule.
3. Add a method to CronJob to get future run times in future.
4. Move should_run_now method to CronJob to override it if required.

[coveralls]

Coverage decreased (-15.0%) to 76.231% when pulling f04caf5 on peynaj:master into 7fa5c2d on Tivix:master.