Fabfile improvements

- Remove gratuitous Unixisms so that fabfile will work on Windows
- Docstrings for tasks so `fab --list` is more useful.
- Add `gh_pages` task for publishing to GitHub Pages
  using [ghp-import](https://github.com/davisp/ghp-import)
Merge pull request #1550 from georgevreilly/fabfile-cleanup

Fabfile improvements