Fixes for recursive tree watcher

[FabianKramm]

Hello! This PR fixes 3 problems we encountered in the recursive tree watcher implementation and adds 2 new tests:

• In the recursive tree implementation stopping a child watch will actually unwatch the parent as well, this can be reproduced with the following snippet on MacOS or Windows which uses the recursive tree implementation:

package main

import (
	"fmt"
	"io/ioutil"
	"os"

	"github.com/rjeczalik/notify"
)

func watch(path string) chan notify.EventInfo {
	c := make(chan notify.EventInfo, 1)
	if err := notify.Watch(path+"...", c, notify.All); err != nil {
		panic(err)
	}
	return c
}

func main() {
	os.MkdirAll("./a/b/c", 0775)
	defer os.RemoveAll("./a")

	// watch a child and parent path across multiple channels. This can happen in any order.
	ch1 := watch("./a/b")
	ch2 := watch("./a/b/c")

	// unwatch ./a/b/c -- this is what triggers unwatching of ./a/b as well
	notify.Stop(ch2)

	// fire an event that will never show up because the path a/b is now unwatched
	go func() { ioutil.WriteFile("a/b/c/d", []byte("X"), 0664) }()

	// Never terminates
	fmt.Println(<-ch1)
}

I added a new test called TestStopChild that tests explicitly for this problem.

• The second problem was already described in FSEvents: fix bug where rewatching path fails across channels #155, but is as well a problem in the recursive tree watcher implementation that does not correctly check children nodes for inactive watchpoints. I added a new test called TestAddParentAfterStop that tests explicitly for this problem. With the fixes introduced in this PR, the code snippet in FSEvents: fix bug where rewatching path fails across channels #155 runs fine without any panics
• The third problem is that on calling Stop(), RecursiveUnwatch is never called, because watchIsRecursive(nd) is called after the watchpoint was already removed and hence the check always returns that the watch is not recursive