Markdown support

Or at least some subset of it.

If we want to keep it pure ruby then the obvious choice of parser/renderer is [kramdown](http://kramdown.rubyforge.org/). This is not as fast or as secure as that Redcarpet library github wrote, but thats probably not a problem right now

My instinct would be to create a separate Markdown module to encapsulate everything, making it easier to swap it for a C based renderer in future if needed. Then just store a `body_html` string on the comment objects which gets rendered to the page, alongside the original user-submitted markdown for use when a comment is edited.

I don't know, maybe extracting it to a module is unnecessary.

Thoughts?