1.  Reporting line numbers for errors.
2.  Checking classes of code blocks.
3.  Tidying up omissions found by running against example lesson.

To separate that content from the "Key Points" injected via
_layouts/reference.html.

Template changes to work with validator according to current link validation rules. Also update HTML output.

* Add `.nojekyll` to stop GitHub trying to compile.
* Get rid of Jekyll `_config.yml`.
* Create placeholder `css` and `img` directories.
* Create `pages` directory with source for web pages.
* Move all Markdown files into `pages`.
* Move `tools` into `pages` (because that's where people will mostly work).
* Move `_includes` and `_layouts` into `pages` (because that's where we'll compile).

This is up for discussion, but at the moment is the
minimal set of files needed to be a Software Carpentry lesson.