add support for pathlib objects, improve logging, fixes #187

[fthiery]

It would be good that pyinotify works with pathlib Path objects (see https://docs.python.org/3.8/library/pathlib.html); currently, the behaviour is that the path is silently dropped, which makes it quite hard to debug.

This MR suggests:

• casting to string whenever possible (makes pathlib objects work)
• adds a more explicit error when not possible to cast to string