OS X Support

Hello,

I'm trying to use pghoard on OS X and it barfs on startup:

```
2018-01-14 21:11:29,959 pghoard MainThread DEBUG Loading JSON config from: 'pghoard.json', signal: None
Traceback (most recent call last):
File "/usr/local/bin/pghoard", line 11, in
sys.exit(main())
File "/usr/local/lib/python3.6/site-packages/pghoard/pghoard.py", line 573, in main
pghoard = PGHoard(config_path)
File "/usr/local/lib/python3.6/site-packages/pghoard/pghoard.py", line 79, in __init__
self.inotify = InotifyWatcher(self.compression_queue)
File "/usr/local/lib/python3.6/site-packages/pghoard/rohmu/inotify.py", line 58, in __init__
self.libc = ctypes.CDLL("libc.so.6", use_errno=True)
File "/usr/local/Cellar/python3/3.6.4_2/Frameworks/Python.framework/Versions/3.6/lib/python3.6/ctypes/__init__.py", line 348, in __init__
self._handle = _dlopen(self._name, mode)
OSError: dlopen(libc.so.6, 6): image not found
```

It looks like you're trying to directly load libc to use Linux's inotify APIs. Obviously that won't work on OS X. I'm not a pythonista so this question may be naive, but have you considered swapping out this custom code with a cross-platform library, like https://pypi.python.org/pypi/watchdog?