Switch to Jekyll for the site generation.

Todo: Fix `/downloads`, `/about` and `/changelog` pages.
Convert to Kramdown definition list
Converted downloads page
Converted About page
WIP: Convert Changelog
Note: trac links are not yet fixed
style.css: Add `padding-bottom` to container.

Fixes issue in some news posts.
Clean up unused selectors.
Switch to Grunt for building everything.
Implement filename-based cache busting for css and js files.
Fix changelog.md trac links.