Feature/markdown preview

fixes #133
closed #1195 in favor of a back-end approach.

More detailed description and screenshots here: http://www.reddit.com/r/ideasfortheadmins/comments/2oaw31/markdown_preview_for_comments_and_self_posts/

Using a tab-base UI approach similar to the github preview would probably be preferable. 

(this PR is a re-issue of #1201