Comments

Resolves #14 and #15. Nested comments with markdown support. :smiley: 
What do you think about moving this to the comment model?  View helpers are kinda gross as you can see here when trying to refactor the markdown render code.  Possibly:

```ruby
class MarkdownRenderer
  def initialize(raw)
    @raw = raw
  end

  def to_html
    # ... turn @raw into html
  end
end

class Comment

  # ....

  def to_html
    @to_html ||= MarkdownRenderer.new(body).to_html
  end

end
```