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"
I think this tries to replicate what Pelican can already do with `EXTRA_PATH_METADATA` as [explained in the docs](http://docs.getpelican.com/en/latest/settings.html#path-metadata). I understand is may seem simpler to have just an `extras` folder, but this would add additional code which would have to be maintained and is not entirely necessary or versatile.