This means that you can trigger div creation by

1. Using a special title
2. Using a special class on an arbitrary title

- Use UPPERCASE for global constants
- Add Learning Objectives to SPECIAL_TITLES
- Fix samples

Previously, Div creation was triggered by a Blockquote that began with a
header that contained a single class that was in a list of special
classes.

Now, Div creation is *also* triggered by a Blockquote that begins with a
header that has it's text contained in a list of special titles. The
title is used to lookup an appropriate class to give the Div.

In particular, 'prerequesites' is a special title, giving the class
'prereq'.

This input:

    > ## Prerequesites
    >
    > A short paragraph describing what learners need to know before
    > tackling this lesson.

will trigger this output:

    <div id="prerequisites" class="prereq">
    <h2>Prerequisites</h2>
    <p>A short paragraph describing what learners need to know before
    tackling this lesson.</p>
    </div>

and others small fixes.

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').

* 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.