Conversion of documentation to yuidoc.

As discussed with Ash, I've migrated and edited some of the documentation to be compatible with yuidoc (http://yui.github.com/yuidoc/syntax/index.html), which supports full markdown and more programmatic documentation.

You can see the generated documentation here:
http://j.shirley.im/tech/yui/gallery-markdown/docs/classes/Markdown.html

(It's all node-based, very easy to setup).

Thanks!