Working speaker notes for slides

I have implemented a `writer` inherited from `FileWriter` which provides the feature of speaker notes in the slideshows loaded by the CDN (it is also functional with the slides using a local copy of reveal.js).
The writer fetch the CDN content of two needed files, and write them to the slideshow directory, to be properly served and then functional. 

To use it, the user has to add the `--notes` flag in the command line, ie:

`ipython nbconvert your_slides.ipynb --to slides --notes --post serve` if the user want to add the possibility to use speaker notes... 

Cheers.

Damián.