Use Jekyll for templating

This pull request moves the site from the node-based custom build system to Jekyll, which is baked directly into GitHub Pages. The big benefit here (beyond not reinventing the wheel), is that you no longer have to commit the compiled site to the repository, thus pull requests to markdown instantly update the site, with no technical knowledge or need to run locally required.  Specifically:

* Move chapters from `/md` to `_chapters`
* Move templates to `_layouts` (note: I kept them as two different templates, but believe in practice they are identical)
* DRY things up by moving header/footer to partials (`_includes`) Note: they were a bit out of sync
* Use YAML frontmatter to store document metadata (e.g., title)
* Dynamically generate table of contents (summary) since chapters are now machine-readable
* Use bower to version vendored assets (bootstrap, jquery, modernizr)
* Use Pygments for syntax highlighting
* Minor markdown changes to better adhere to the spec (e.g., wrapping special characters in code blocks, colons before footnote definitions)
* Add sitemap for better SEO


The output should remain nearly identical, and there should be no substantive changes to the content. Once merged, potential contributors can propose changes to content by simply modifying the files int he `_chapters` folder, with no need to run locally or risk that they edit the output, etc.