Added RemoveFunc

With some test code:
```go
package main

import (
	"fmt"
	"cron"
	"time"
)

func main() {
	c := cron.New()
	j1, _ := c.AddFunc("* * * * * *", func() { fmt.Println("1") })
	j2, _ := c.AddFunc("* * * * * *", func() { fmt.Println("2") })
	j3, _ := c.AddFunc("* * * * * *", func() { fmt.Println("3") })
	fmt.Println(j1, j2, j3)
	c.RemoveFunc(j2)
	c.Start()
	time.Sleep(time.Second * 5)
	c.RemoveFunc(j1)
	select {}
}
```
I'm sorry I don't know how to exclude the .gitignore from the pull request, please just ignore it.