Add package-level docstrings for sphinx docs
Fix RST syntax in docstrings.
Switch to sphinx RTD theme.

Conditionally add the sphinx RTD theme if we're not on RTD.
Properly wordwrap the sphinx README.md
Add sphinx-apidoc generated autosummary docs

Document stubs provide documentation for the entire project.
Merge pull request #297 from jcrobak/sphinx-improvements

sphinx documentation improvements
Merge branch 'master' of https://github.com/spotify/luigi