Make filter for id at definitions general

This solves #77 by add id attribute to every item of a definition.
Setting layout for LICENSE correctly
Correcting URLs in footer.

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.
Add requirements.txt necessary to build and run validator
Merge pull request #98 from gvwilson/adding-url-placeholder

Adding url placeholder
Merge pull request #81 from r-gaia-cs/fix-glossary

Make filter for id at definitions general
Merge pull request #102 from abought/add_requirements_core

Add requirements.txt necessary to build and run validator
Merge branch 'core' into gh-pages