Improve logging of YAML header errors

added some challenge CSS

Make validator imports more pythonic

Fix typo

Update License.md md5sum on tools/check based on 2f4703e8

Add requirements.txt necessary to build and run validator

Make filter for id at definitions general

Adding url placeholder

The original plan was to put the URL for this repository in a configuration file that authors could edit when creating actual lessons.  However, it looks like Pandoc only does one level of variable expansion, so variables inside `_includes/footer.html` (which is itself interpolated as a variable) aren't expanded.  Instead, we will tell authors to edit `_includes/footer.html` in the `README.md` in the `gh-pages` branch.

Fix unittest for changes on motivation.md

This solves #77 by add id attribute to every item of a definition.

Add Unit Test target to Makefile

Test rework

Improve message for glossary error

Minor changes to make error messages consistent, and possibly less crashy. Still need unit tests to trigger this code.

fill in missing '$ ' in prompt

Improve callout box and "learning objectives" box

Add small script to setup label in lesson repo

Update validator for changes in directory structure

Updating files to move  into root directory