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)
- Automatically copy contents of "extras" directory (if it exists)
  to DEPLOY_PATH. Put favicon.ico and CNAME (for GitHub Pages) in "extras"