Fixed reveal in the new nbconvert to make it work.

OK, here it is the changes to make reveal work in the new nbconvert...

The fixes:
* Change in reveal.tpl: 
    * Fixed paths to reveal.js folder
    * Change location of mathjax config file
    * Add mathjax onload in the template (to avoid mathjax rendering bad, we render at the end).
    * Hide solcial buttons (to have a simpler slideshow, let the user complicate it!)
* Changes in revealhelp.py to fix a bug when there is no slideshow metadata in a cell.
* Changes in exporter/reveal.py to register the CSSHtmlHeaderTransformer

OK, now reveal slideshow is working... I hope.

Cheers.

Damián.