Improving setup instructions

Multiple changes based on converting Make lesson to template

1.  Moving the build steps up to the top of `README.md`.
2.  Removed the misleading target to run the checking tool (since the tool doesn't actually work).

And taking it out of the auto-displayed list of available commands.

[RDY] pandoc filter to convert blockquotes to divs

This filter converts all blockquote-with-attributes to
div-with-attributes.

A blockquote-with-attributes is defined as a blockquote that has a
header with attributes defined on it as it's first element, e.g.

    > # valid blockquote {.class}
    > text

This example would be converted into this markdown:

    <div class='class' id=id>
    # valid blockquote
    text
    </div>

And the equivalent html.

The class must be one of the allowed classes defined in the filter (i.e.
one of 'callout', 'objectives' or 'challenge').

Adding missing CSS files.

Fix layout to follow bc CSS

Adding slides

Also modifed the Makefile to use the slides template for this file
instead of the generic template.

Note that we do not need a Markdown extension for deck.js because
Pandoc is doing Markdown-to-HTML conversion for us.

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.

Include standard page layout

The HTML files in the root directory of this repository don't belong in version control,
since they are built from the demonstration files in `pages` rather than from real lessons.
However, the HTML files in the root of actual lessons definitely *do* belong in the repo,
so the `.gitignore` shouldn't ignore them.

1. Modify `.gitignore` to only ignore HTML files in the current directory.
2. Add `_includes/page.html` template.

Putting template into final [sic] form

1. Moving all commands into a Makefile (because we're handling file dependencies, and that's the right tool for it).
2. Ignoring the .html files produced in the root directory for now.
3. Switching to a single layout modeled on that used for workshop websites - will need to update this.

* 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).

Adding a note to remind authors to say what learners are supposed to do in lessons.

LICENSE.md: Bump the CC BY 3.0 to 4.0

Titles from the v4 release notes [1]:

* A more global license
* Rights outside the scope of copyright
* Common-sense attribution
* Enabling more anonymity, when desired
* 30-day window to correct license violations
* Increased readability
* Clarity about adaptations

[1]: https://creativecommons.org/version4

Using template variables everywhere

- Fixed error in header.html; missing }