Markdowned elements in issues and comments are not styled

It's great to be able to use markdown in issues and comments, but the css reset left some elements unstyled.
I added some basic style to `ul`, `ol`, `pre` and `code` (for these last two, the style is from GitHub).
Tested via browserstack on IE6+, FF3.6+, Cr, Opera10+, Safari4+, iOS6