func (e *Entry) Do(job Job) {
	e.Job = job
}
Do adds a Job to the Entry.
package clockwerk
import (
	"time"
)
type Job interface {
	Run()
}
type Entry struct {
	Period time.Duration
	Next   time.Time
	Prev   time.Time
	Job    Job
}
func newEntry(period time.Duration) *Entry {
	return &Entry{
		Period: period,
		Prev:   time.Unix(0, 0),
	}
}