fix the issue: watch & unwatch the same folder on Windows

[Falldog]

fix the issue : watch & unwatch the same folder on Windows may cause exception "Access Denied"
should close the dir handle

testing code

import os
import sys
import time
import shutil
import logging
import tempfile
import threading
from os.path import join, dirname, abspath
from watchdog.observers import Observer
from watchdog.events import LoggingEventHandler

class ObserverThread(threading.Thread):
    def __init__(self, folder):
        threading.Thread.__init__(self)
        self.folder = folder
        self.stop_evt = threading.Event()
    def stop(self):
        self.stop_evt.set()
    def run(self):
        event_handler = LoggingEventHandler()
        observer = Observer()
        observer.schedule(event_handler, self.folder, recursive=True)
        observer.start()
        self.stop_evt.wait()
        observer.stop()
        observer.join()

def main():
    logging.basicConfig(level=logging.INFO, format='%(asctime)s - %(message)s', datefmt='%Y-%m-%d %H:%M:%S')
    folder = tempfile.mkdtemp()    # watch & unwatch the same folder 10 times
    for i in range(10):
        if not os.path.exists(folder):
            os.mkdir(folder)
        tmp_file = join(folder, 'tmp.txt')
        print 'process %d temp_file=%s' % (i, tmp_file)

        t = ObserverThread(folder)
        t.start()
        with open(tmp_file, 'w') as f:
            f.write('abc')
        os.remove(tmp_file)
        t.stop()
        t.join()

        shutil.rmtree(folder)

if __name__ == "__main__":
    main()