Offline jQuery / Zepto

Fixes #93.

This PR bundles ZeptoJS (a smaller, less backwards-compatible version of jQuery) inline, so that the stack works offline, too. Much easier to do than writing all of this in pure JavaScript (yuck).

While I was at it, I also fixed (I hope?) the scroll-to-top feature, that was trying to use a string value and only not taking padding etc. into account.

The syntax highlighter will not work offline, but that isn't a crucial feature, so I don't care.