Update task tutorial

Update task tutorial (#2)

* Add task package.

* Update task package.

* Make task package works as expected.

* Add task examples.

* Add task stats and error handler.

* Add Coordinator.

* Update coordinator.

* Bugs fix.

* Add more examples.

* Some fixes.

* Add test cases.

* Add go.sum.

* Change coordination implementation to setnx.

* Fix no at time periodic schedule.

* Limit job execution time; limit worker count.

* Add jobLock, add Remove and Clear job methods in scheduler.

* Add ClearJobs; Update OnceJob scheduler implementation.

* Add redis cluster support for coordinator.

* Move Coordinate to Run; add unit tests for Coordinate.

* Extract commonJob from OnceJob and PeriodicJob.

* unexport cron expression type.

* Add JobCount.

* Add timezone support in cron.

* Add Stop for Scheduler.

* Add coordinate for oncejob.

* extract coordinate to commonJob.

* Update unit tests.

* Add GetLatestScheduledTime.

* Move unit test into a separate package.

* Add start flag for scheduler.

* make some functions and methods unexported.

* Add examples for task.

* Change IntervalType to cronIntervalType.

* Add comments for exported items.

* Add task tutorial.

* Update README.

* Update README.

* Update go.sum.

* Update task tutorial.

Add task package

Add task package (#1)

* Add task package.

* Update task package.

* Make task package works as expected.

* Add task examples.

* Add task stats and error handler.

* Add Coordinator.

* Update coordinator.

* Bugs fix.

* Add more examples.

* Some fixes.

* Add test cases.

* Add go.sum.

* Change coordination implementation to setnx.

* Fix no at time periodic schedule.

* Limit job execution time; limit worker count.

* Add jobLock, add Remove and Clear job methods in scheduler.

* Add ClearJobs; Update OnceJob scheduler implementation.

* Add redis cluster support for coordinator.

* Move Coordinate to Run; add unit tests for Coordinate.

* Extract commonJob from OnceJob and PeriodicJob.

* unexport cron expression type.

* Add JobCount.

* Add timezone support in cron.

* Add Stop for Scheduler.

* Add coordinate for oncejob.

* extract coordinate to commonJob.

* Update unit tests.

* Add GetLatestScheduledTime.

* Move unit test into a separate package.

* Add start flag for scheduler.

* make some functions and methods unexported.

* Add examples for task.

* Change IntervalType to cronIntervalType.

* Add comments for exported items.

* Add task tutorial.

* Update README.

* Update README.

* Update go.sum.