avoid overwriting another user's database

Should fix issue #39

autojump

@@ -49,6 +49,9 @@ def dicadd(dic, key, increment=1):
 def save(path_dict, dic_file):
     """Save the database in an atomic way, and preserve
        a backup file."""
+    # If the dic_file exists, check that it belongs to us
+    # Otherwise, fail quietly
+    if (not os.path.exists(dic_file)) or os.getuid() == os.stat(dic_file)[4]:
         temp = NamedTemporaryFile(dir=CONFIG_DIR, delete=False)
         pickle.dump(path_dict, temp, -1)
         temp.flush()