gh-pages.sh -- jekyll_site_directory: Correct path in value

- Was missing a forward slash
gh-pages.sh -- rsync_opts: Change exclude options

It's sensible to use .gitignore as a rsync exclude file:
- Delete "--exclude=...", leaving only "--exclude=.git*"
- Add option "--exclude-from=${tmp_dir}/.gitignore"
gh-pages.sh: Manually copy .gitignore back and forth

Since we never pull from other branches to gh-pages, .gitignore doesn't
get updated. We need to do it manually, so that files we've ignored in
the master branch don't get added in dh-pages, and also because we're
now using .gitignore as a rsync exclude file.