[core] fix #1176 sync check with HTTP long polling

Set client configuration option `sync` to enable auto sync.

Example
```javascript
var options = {
    url  : 'php/connector.php',
    sync : 10000  // Checks every 10sec
}
$('#elfinder').elfinder(options);
```

And added new volume driver root options this time.

```php
// Is support parent directory time stamp update on add|remove|rename item
// Default `null` is auto detection that is LocalFileSystem, FTP or
// Dropbox are `true`
'syncChkAsTs'  => null,

// Long pooling sync checker function for syncChkAsTs is true
// Calls with args (TARGET DIRCTORY PATH, STAND-BY(sec), OLD TIMESTAMP, VOLUME DRIVER INSTANCE, ELFINDER INSTANCE)
// This function must return the following values. Changed: New Timestamp or Same: Old Timestamp or Error: false
// Default `null` is try use
// elFinderVolumeLocalFileSystem::localFileSystemInotify() on LocalFileSystem driver
// another driver use elFinder stat() checker
'syncCheckFunc'=> null,

// Long polling sync stand-by time (sec)
'plStandby'    => 30,

// Sleep time (sec) for elFinder stat() checker (syncChkAsTs is true)
'tsPlSleep'    => 10,

// Sleep time (sec) for elFinder ls() checker (syncChkAsTs is false)
'lsPlSleep'    => 30,

// Client side sync interval minimum (ms)
// Default `null` is auto set to ('tsPlSleep' or 'lsPlSleep') * 1000
'syncMinMs'    => null,
```

php/elFinderVolumeLocalFileSystem.class.php

@@ -76,6 +76,14 @@ class elFinderVolumeLocalFileSystem extends elFinderVolumeDriver {
 		if (!empty($this->options['startPath'])) {
 			$this->options['startPath'] = $this->getFullPath($this->options['startPath'], $cwd);
 		}
+		
+		if (is_null($this->options['syncChkAsTs'])) {
+			$this->options['syncChkAsTs'] = true;
+		}
+		if (is_null($this->options['syncCheckFunc'])) {
+			$this->options['syncCheckFunc'] = array($this, 'localFileSystemInotify');
+		}
+		
 		return true;
 	}
 	
@@ -148,6 +156,56 @@ class elFinderVolumeLocalFileSystem extends elFinderVolumeDriver {
 		}
 	}
 	
+	/**
+	 * Long pooling sync checker
+	 * This function require server command `inotifywait`
+	 * If `inotifywait` need full path, Please add `define('ELFINER_INOTIFYWAIT_PATH', '/PATH_TO/inotifywait');` into connector.php
+	 * 
+	 * @param string     $path
+	 * @param int        $standby
+	 * @param number     $compare
+	 * @return number|bool
+	 */
+	public function localFileSystemInotify($path, $standby, $compare) {
+		if (isset($this->sessionCache['localFileSystemInotify_disable'])) {
+			return false;
+		}
+		$path = realpath($path);
+		$mtime = filemtime($path);
+		if ($mtime != $compare) {
+			return $mtime;
+		}
+		$inotifywait = defined('ELFINER_INOTIFYWAIT_PATH')? ELFINER_INOTIFYWAIT_PATH : 'inotifywait';
+		$path = escapeshellarg($path);
+		$standby = max(1, intval($standby));
+		$cmd = $inotifywait.' '.$path.' -t '.$standby.' -e moved_to,moved_from,move,create,delete,delete_self';
+		$o = $r = '';
+		$this->procExec($cmd , $o, $r);
+		if ($r === 0) {
+			// changed
+			clearstatcache();
+			return filemtime($path);
+		} else if ($r === 2) {
+			// not changed (timeout)
+			return $compare;
+		}
+		// error
+		// cache to $_SESSION
+		$sessionClose = true;;
+		try {
+			$sessionStart = session_start();
+		} catch (Exception $e) {
+			$sessionClose = false;
+		}
+		if ($sessionStart) {
+			$this->sessionCache = &$_SESSION[elFinder::$sessionCacheKey][$this->id];
+			$this->sessionCache['localFileSystemInotify_disable'] = true;
+			$sessionClose && session_write_close();
+		}
+		
+		return false;
+	}
+	
 	/*********************************************************************/
 	/*                               FS API                              */
 	/*********************************************************************/