Fix regression on Mac

This reverts commit fe451b757e0864639752605c163ebb0d11efe492.

Great to see people are finally working on this repo and keeping up to date! Commit fe451b757e0864639752605c163ebb0d11efe492 resulted in the following error messages after calling `observer.stop()` in my code:
```
python[11364] (CarbonCore.framework) FSEventStreamStop(): failed assertion 'streamRef != NULL'
python[11364] (CarbonCore.framework) FSEventStreamInvalidate(): failed assertion 'streamRef != NULL'
python[11364] (CarbonCore.framework) FSEventStreamRelease(): failed assertion 'streamRef != NULL'
/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.py:604: RuntimeWarning: tp_compare didn't return -1 or -2 for exception
  del _active[_get_ident()]
```
I'm using Mac OS X 10.8.4. Reverting the commit fixes the problem. I'm too tired right now to figure out exactly why the commit was causing problems. :)