Add a __sub__ implementation to DirectorySnapshot

Signed-off-by: Gora Khargosh <gora.khargosh@gmail.com>
diff --git a/watchdog/dirsnapshot.py b/watchdog/dirsnapshot.py
index cd117e5..5a360ae 100644
--- a/watchdog/dirsnapshot.py
+++ b/watchdog/dirsnapshot.py
@@ -67,6 +67,8 @@
                 directory_path = path_join(root, directory_name)
                 self._dirs_set.add(directory_path)
 
+    def __sub__(self, previous_dirsnap):
+        return DirectorySnapshotDiff(previous_dirsnap, self)
 
     @property
     def stat_snapshot(self):