Update the bundled Markdown lib to version 1.4.

Any interest in merging in any of these changes that we are running on our fork of this?

* Update bundled Markdown Lib to [version 1.4](http://michelf.ca/projects/php-markdown/), it's PHP 5.3 only.
* Run all content through the parser, not just content that begins with `\^md\s+`.
* Re-enable the built in P2 checkbox functionality
* hook in on priority 2, fixes an issue with the first P2 checkbox not working