a bunch of things

[JohnRoesler]

What does this do?

• renamed job.Err to job. Error
• remove, removeAfterLastRun - just remove when a job hits the limit by default. we don't have any way to restart a job, so there is not point in keeping them in the scheduler once they're stopped
• add milliseconds explicitly as a unit (already available using the Every(time.Duration) option)
• allow At() to accept an interface, string or time.Time (parsing hours down if time.Time)

Which issue(s) does this PR fix/relate to?

List any changes that modify/break current functionality

Have you included tests for your changes?

Did you document any new/modified functionality?

• Updated example_test.go
• Updated README.md

Notes