Valid XHTML 1.1! (well... almost...)

I've modified my wiki so that it is now generating valid XHTML 1.1 code with a correct MIME type of "application/xml+xhtml". Some of the code are still invalid. These are mainly related to the nested list.

Also, I've remove all the tables for layout. Now the code using CSS is much cleaner while the layout look more or less the same!