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

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

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`.

Closes #75.

1.  Created `bin/repo_check.py`.
2.  Moved `require()` to `util.py`.
3.  Updated `Makefile` with new target.

add function knitr_fig_path