Rq

- `Worker` statistics! `Worker` now keeps track of `last_heartbeat`, `successful_job_count`, `failed_job_count` and `total_working_time`. Thanks selwin!
- `Worker` now sends heartbeat during suspension check. Thanks theodesp!
- Added `queue.delete()` method to delete `Queue` objects entirely from Redis. Thanks theodesp!
- More robust exception string decoding. Thanks stylight!
- Added `--logging-level` option to command line scripts. Thanks jiajunhuang!
- Added millisecond precision to job timestamps. Thanks samuelcolvin!
- Python 2.6 is no longer supported. Thanks samuelcolvin!

* Fixed lock management error. Thanks chaffeqa!

- Fixed an issue where `job.save()` may fail with unpickleable return value.

- Replace `job.id` with `Job` instance in local `_job_stack `. Thanks katichev!
- `job.save()` no longer implicitly calls `job.cleanup()`. Thanks katichev!
- Properly catch `StopRequested` `worker.heartbeat()`. Thanks fate0!
- You can now pass in timeout in days. Thanks yaniv-g!
- The core logic of sending job to `FailedQueue` has been moved to `rq.handlers.move_to_failed_queue`. Thanks yaniv-g!
- RQ cli commands now accept `--path` parameter. Thanks kirill and sjtbham!
- Make `job.dependency` slightly more efficient. Thanks liangsijian!
- `FailedQueue` now returns jobs with the correct class. Thanks amjith!

- Refactored APIs to allow custom `Connection`, `Job`, `Worker` and `Queue` classes via CLI. Thanks jezdez!
- `job.delete()` now properly cleans itself from job registries. Thanks selwin!
- `Worker` should no longer overwrite `job.meta`. Thanks WeatherGod!
- `job.save_meta()` can now be used to persist custom job data. Thanks katichev!
- Added Redis Sentinel support. Thanks strawposter!
- Make `Worker.find_by_key()` more efficient. Thanks selwin!
- You can now specify job `timeout` using strings such as `queue.enqueue(foo, timeout='1m')`. Thanks luojiebin!
- Better unicode handling. Thanks myme5261314 and jaywink!
- Sentry should default to HTTP transport. Thanks Atala!
- Improve `HerokuWorker` termination logic. Thanks samuelcolvin!

- Fixes a bug that prevents fetching jobs from `FailedQueue` (765). Thanks jsurloppe!
- Fixes race condition when enqueueing jobs with dependency (742). Thanks th3hamm0r!
- Skip a test that requires Linux signals on MacOS (763). Thanks jezdez!
- `enqueue_job` should use Redis pipeline when available (761). Thanks mtdewulf!