added new prefixes to classes

With the upcoming 8.0 version of [highlight.js](http://highlightjs.org/ "Syntax highlighting for the Web"), the old stylesheets will no longer work without the new `.hljs` prefixes. See also [#298](https://github.com/isagalaev/highlight.js/pull/298 "Span Class Prefixing") and [#7b48d7f](https://github.com/isagalaev/highlight.js/commit/7b48d7f "Merge branch 'class-prefix'").

* classes are translated to ones with prefixes.
* changed all pre code to `.hljs`
* removed all the `pre` tags
* followed the new styling rules: most noticeable being the need for newlines per selector group
* all the `.base16-*` classes, which will not be used anymore,  are turned into comments