Improve logging of YAML header errors

added some challenge CSS

Make validator imports more pythonic

Fix typo

Merge #108 into gh-pages

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

Add requirements.txt to gh-pages branch

Add requirements.txt necessary to build and run validator

Make filter for id at definitions general

Adding url placeholder

Add links in reference back to sections

Actually add requirements to file

Add info to README.md

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.

remove layout, faq, design html

As of pull #94, LAYOUT.html, FAQ.html, DESIGN.html, and README.html are
no longer produced by the Makefile. That pull should also have removed
the .html files (README.html was already removed in 1c096c78).

Don't generate README.html (and others)

readme: link to new pandoc.org website

Uses the filter-out Make text function

"Returns all whitespace-separated words in text that do not match any of
the pattern words, removing the words that do match one or more."

http://www.gnu.org/software/make/manual/make.html#Text-Functions