Add "formatting help" link with markdown info

We've done the work to implement markdown, so let's actually make it more usable. I suggest we add a little link next to the "post comment" button under the text box on the discussion panel that says "formatting options" or "formatting help" which pops up a little window describing the markdown syntax options. This can be as simple as a popup with a text description of how it works and a list of syntax options. In the future I'd love to see user friendly formatting buttons, but for now I think a little help link will go a long way.