Add blank lines before code blocks

"Code blocks need to have a blank line before them."
https://help.github.com/articles/github-flavored-markdown#fenced-code-blocks
Remove trailing whitespace
Use Jekyll's native templating and rendering

This removes the need for a build script; everything is handled by
GitHub Pages.
Remove jQuery and use plain JavaScript instead

No sense including a 92KB library on a 96KB website.
Add a .gitignore for Jekyll's `_site` directory
Remove redundant `publish.sh` (#19)
Fix image link for README.md