Ignore -1 watch descriptor value.

If wd == -1 this code throws exception, so just ignore this value

```
Traceback (most recent call last):
  File "/usr/local/lib/python2.6/dist-packages/watchdog/observers/api.py", line 196, in run
    self.queue_events(self.timeout)
  File "/usr/local/lib/python2.6/dist-packages/watchdog/observers/inotify.py", line 751, in queue_events
    inotify_events = self._inotify.read_events()
  File "/usr/local/lib/python2.6/dist-packages/watchdog/observers/inotify.py", line 541, in read_events
    wd_path = self._path_for_wd[wd]
KeyError: -1
```