Use a CDN to get reveal.js library.

To get some background about the discussion see: https://github.com/ipython/ipython/issues/3499

This PR makes reveal.js load from a CDN kindly provided by jsdelivr (I submitted a PR there to incorporate all the reveal.js necessary files and they merge it today).

So, now the reveal converter it fully functional in his basic implementation (NOTE, I am laying to you because the speaker notes are not working until I get the new Writer API that @jdfreder is adding to IPython.nbconvert, but ALL the other features are covered and working!).

I think this PR resolve the discussion we had previously and provides to the users with a static slideshow capability directly from the notebook with only two step:

`ipython nbconvert reveal your_slideshow.ipynb`

`python -m SimpleHTTPServer 8000`

We can even simplify this with a command like `--serve` to automate the second step...

* Addendum: I make the prefix of the url to get reveal configurable, so, by default it will use the CDN from jsdelivr, but if the user want to use a local copy of reveal, s/he only has to change the url_prefix in the configurable object.

Cheers.

Damián.