Close #178

Jekyll doesn't export the _episodes_rmd so we need to flag the files.
Each Rmd file must have in the YAML header

    source: Rmd

https://github.com/swcarpentry/lesson-example/pull/104#issuecomment-283310978

1. Renaming episode_title to episode_navbar since it now has dual use.
2. Removing hr elements that look odd with footer nav arrows.
3. Conditionalizing display of episode title.
4. Modifying layouts for breaks and episodes to include the footer nav arrows.

1.  Put prev/next finding code into separate include.
2.  Modify episode_title to use that.
3.  Include it in the footer and add prev/next arrows if there's a match.

Note: the prev/next finding code must be added to *both* episode_title and footer because variables set in includes aren't exported to other includes.

The original plan was to put the URL for this repository in a configuration file that authors could edit when creating actual lessons.  However, it looks like Pandoc only does one level of variable expansion, so variables inside `_includes/footer.html` (which is itself interpolated as a variable) aren't expanded.  Instead, we will tell authors to edit `_includes/footer.html` in the `README.md` in the `gh-pages` branch.

* 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.