Add classes

All classes **must** be explicit because this will make easy
to translate the lessons.

Add input cell question to FAQ

Small improvements to CSS

Close #116.

Link text validation changes

Changes to link text validation per #58 and #90. Validate link text in index.md (always), or only in headings (reference.md and instructors.md)

Demote msg level when http links skipped

Strip anchor from filenames in error messages

Validator logging QA bugfixes

-   Remove extra space before title of boxes
-   Remove background color of boxes when printing
-   Add "Input", "Output" and "Error" tips for code blocks
    when printing
-   Avoid duplicate lines

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