Make setup.py build the docs

[Jason Gerard DeRose]

Not sure exactly what the best practice is here, but probably looking at the way bzr does it would be a good place to start.

I believe this will require a custom distutils command, similar to the Test command we already have. There are two approaches you can take to building the docs. From the source tree root, you can run:

  sphinx-build -b html doc/ doc/_build/

Or from doc/ you can run:

  make html

The first is probably better as it should work fine on Windows also. We want our daily builds and stable releases to build the docs using setup.py, and we'll add a `python3-filestore-doc` package.