Markdown => HTML in 'body' and 'description'

`kramdown` is used to convert the markdown. However the `govspeak` gem is included
in the dependency, because HTML sanitisation will use this gem.
Document markdown => html conversion in README
Merge pull request #35 from alphagov/allow-bodies-and-descriptions-to-contain-markdown

Markdown => HTML in 'body' and 'description'