- generated markup is now well formed XML

Hi there,

thanks for your great work, I really enjoy being able to export org to reveal.js. 

I've made a few small changes to make the markup well formed, in case you're interested.

Cheers,

-Torstein


Changes:

- generated markup is now well formed XML

- XML doc types should be uppercase.

- added a few new lines so that all elements are on their own line