Multiple changes based on converting Make lesson to template

Converted the existing lesson Make to the new template - result is in https://github.com/gvwilson/make.  Learned a few things from doing that, so updated the lesson template:

1. Removed requirement for keywords in index page.  (We'll wait until we actually know how we want to do this before requiring it.)

2. Added a discussion page for advanced notes, pointers to other material, etc.

3. Settled on a single page layout `_layouts/page.html` with `title` and `subtitle` fields for all pages other than the slides.

4. Added a target to the Makefile to delete generated HTML (@aaren was right...).

5. Rewrote parts of `README.md` to describe these changes.

6. Settled on using **bold face** to mark glossary entries.

7. Used four-space indent everywhere in our Markdown.
And what about using [article](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/article) tag in the template:

~~~
<div class="span10 offset1">
<h1 class="title">$title$</h1>
<article>
$if(subtitle)$<h1 class="subtitle">$subtitle$</h1>$endif$
$body$
</article>
</div>
~~~