Documentation on 'Read the Docs'

I have added a pip-style dependencies file, so we can have the documentation created and kept up-to-date by readthedocs.org.
Without that file, readthedocs will not supply pathtools to the documentation, resulting in empty API documentation for most classes.

You can see the result at: https://watchdog.readthedocs.org/en/latest/index.html
Unsurprisingly, it builds the correct documentation. That page is rebuilt nightly, so the documentation there is always fresh. Alternatively, you can add http://readthedocs.org/build/5593 to your commit hooks. 

If you merge this pull request, I will change the readthedocs page to point to your repository, or hand over the readthedocs configuration at your choice.