Fixed stop hang & stop err

In the follow case, `Stop` will hang:

```
package main

import (
	"fmt"
	"github.com/ActiveState/tail"
	"sync"
	"time"
)

func main() {
	t, err := tail.TailFile("/tmp/no.log", tail.Config{
		Follow:    true,
		MustExist: false,
	})
	if err != nil {
		fmt.Println(err)
		return
	}

	CheckTick := time.Tick(2 * time.Second)
	exit := make(chan bool)
	wg := sync.WaitGroup{}
	wg.Add(1)
	go func() {
		defer wg.Done()
		for {
			select {
			case l := <-t.Lines:
				fmt.Println(l)
			case <-exit:
				return
			case <-CheckTick:
				fmt.Println("hang on stop")
				err := t.Stop()
				fmt.Printf("stop done, err = %v\n", err)
				t1, _ := tail.TailFile("/tmp/no1.log", tail.Config{
					Follow:    true,
					MustExist: false,
				})
				t = t1
			}
		}
	}()
	time.Sleep(3 * time.Second)
	exit <- true
	wg.Wait()
	fmt.Println("stop done. Exit now.")
}
```
note: the file will tail not exist but the dir exist.

In this case, [`BlockUntilExists`](https://github.com/ActiveState/tail/blob/master/watch/inotify.go#L48) will wait `Event` or `Dying()`, but the param is a struct, so the `tomb.Tomb` in this method is a copy value and [`Daing()`](http://bazaar.launchpad.net/~niemeyer/tomb/trunk/view/head:/tomb.go#L101) will call [`init()`](http://bazaar.launchpad.net/~niemeyer/tomb/trunk/view/head:/tomb.go#L82), it will make a *new dying channel* and this channel never closed. As we see, `BlockUntilExists` never return until the log file appear and `reopen` never done, so [`Done`](https://github.com/ActiveState/tail/blob/master/tail.go#L151) never called and `Stop` will hang.