As per swcarpentry/lesson-template#140, but only the core CSS
changes.

1. Bringing in Bootstrap CSS files.
2. Merging swc.css and swc-bootstrap.css from 'bc' repository.
3. Updating _includes/header.html to refer to the right files.

* 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.