Replace inotify with polling observer

I opened #178 to discuss my issues running mkdocs inside Docker/Vagrant setups. After talking to a watchdog developer, he suggested using a ``PollingObserver`` which is a more universal approach but likely also a little bit slower.

I believe these changes will allow auto-reload to work inside shared folders. My brief testing suggests so anyhow. Existing users shouldn't notice any difference.

I absolutely couldn't figure out any sensible way to unit test my four line pull request. I also didn't find any place in the project documentation to write about it.
Let's expand on that slightly.

    # We could have used `Observer()`, which can be faster, but `PollingObserver()` works more universally.