Add prefixfree and adjust CSS

With prefixfree you don't need a devserver or anything to get the same look as
in the version built with gulp.

Changed `columns` to `column-count` to counter prefixfree bug here due to CSS
naming weirdness: https://github.com/LeaVerou/prefixfree/issues/54
Merge pull request #983 from passy/prefixfree

Add prefixfree and adjust CSS