func RetryNotify(operation Operation, b BackOff, notify Notify) error {
	return RetryNotifyWithContext(nil, operation, b, notify)
}
RetryNotify calls notify function with the error and wait duration
for each failed attempt before sleep.
package backoff
import (
	"time"
	"golang.org/x/net/context"
)
type Operation func() error
type Notify func(error, time.Duration)
func Retry(o Operation, b BackOff) error { return RetryNotify(o, b, nil) }