Fixed broken markdown package

Hi David,

some days ago a [user][dartisan] was complaining about a broken [markdown package][pubmarkdown]. So I did some changes to fix all issues preserving your fenced code block extension ala GitHub.

Please merge my changes into the main branch and update your package at [pub.dartlang.org][pubmarkdown].

[dartisan]: https://plus.google.com/u/0/103493864228790779294/posts/GrNnVjiBbzj
[pubmarkdown]: http://pub.dartlang.org/packages/markdown