Fix "br" typo causing ugly rendering (epub only)

</br> instead of <br/> renders fine in the browser but not in an epub

1. Renaming episode_title to episode_navbar since it now has dual use.
2. Removing hr elements that look odd with footer nav arrows.
3. Conditionalizing display of episode title.
4. Modifying layouts for breaks and episodes to include the footer nav arrows.

Closes #35.

1.  Fixing image link in `setup.md`.
2.  Regenerating all-figures.

Closes #42.

An internally-defined Markdown link has the form [xxx][yyy].  If the
ID 'yyy' doesn't resolve, the text is left as-is, so we check for
that, then subtract those that reference configuration values using
'{{'.  It's a hack, but it'll catch at least a few things.

Import future print in case of python2 environment

[fix # 82] Check R packages required by the lesson

Using `site.github.url` to refer to the repository fails to load resources when the GitHub Pages site is accessed via HTTPS, because `site.github.url` is `http` protocol.  This modifies loads of CSS and Javascript files to always use `https`.

Updating CITATION file

Merging new styles and documentation

Closes #75.