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

Only cleaning intermediate R files on demand.

1.  Use site.root for root path if set, otherwise use site.github.url.
2.  Display extra files for workshop-template repo as well as for lessons.

1.  Moving all YAML library usage into 'util.py' via utility function.
2.  Wrapping 'import yaml' in error handling to report what library to install.

1.  Converting error reporting.
2.  Simplifying checking logic.
3.  Refactoring metadata loading.
4.  Modifying lesson checker to reflect refactoring.

Closes #47.