Feature/javascript markdown preview

fixes #133

Also discussed here: [http://www.reddit.com/r/ideasfortheadmins/comments/2oaw31/markdown_preview_for_comments_and_self_posts/](http://www.reddit.com/r/ideasfortheadmins/comments/2oaw31/markdown_preview_for_comments_and_self_posts/)

 This uses the library @gamefreak mentioned. I also implemented this using a backend solution [here](https://github.com/kochis/reddit/pull/1), but the front-end approach seems better.

The UI could probably be improved a bit, I'm not exactly a designer :/