Schedule-cronjob

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- Fixed history (no code changes).

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- This release introduces two new features in a backwards compatible way:
- Allow jobs to cancel repeated execution: Jobs can be cancelled by calling ``schedule.cancel_job()`` or by returning ``schedule.CancelJob`` from the job function. (Thanks to cfrco and matrixise.)
- Updated ``at_time()`` to allow running jobs at a particular time every hour. Example: ``every().hour.at(':15').do(job)`` will run ``job`` 15 minutes after every full hour. (Thanks mattss.)
- Refactored unit tests to mock ``datetime`` in a cleaner way. (Thanks matts.)

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- Fixed an issue with ``next_run()`` throwing a ``ValueError`` exception when the job queue is empty. Thanks to dpagano for pointing this out and thanks to mrhwick for quickly providing a fix.

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- Fixed issue with ``at_time`` jobs not running on the same day the job is created (Thanks to mattss)

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- Added ``schedule.next_run()``
- Added ``schedule.idle_seconds()``
- Args passed into ``do()`` are forwarded to the job function at call time
- Increased test coverage to 100%

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- Changed default ``delay_seconds`` for ``schedule.run_all()`` to 0 (from 60)
- Increased test coverage