Added working speaker notes for slides.

Tomorrow morning I had a discussion with @jdfreder from a previous PR #3852. His comments where high valuable, so I closed this PR in favor of this one with, I think, a better implementation.
To resume, I fetch the cdn containing the files we need to make speaker notes works. Then I pass this info using the resources dict to the write which output the files in the correct path...
I make some other additions to make this feature an option from the command line, ie:

* if you want a simple slideshow:

`ipython nbconvert your_slides.ipynb --to slides`
* if you want a simple **served** slideshow:

`ipython nbconvert your_slides.ipynb --to slides --post serve`
* if you want to use a local copy of the `reveal.js` library:

`ipython nbconvert your_slides.ipynb --to slides --post serve --local reveal.js`
* and if you want to use speaker notes:

`ipython nbconvert your_slides.ipynb --to slides --post serve --notes True`
or
`ipython nbconvert your_slides.ipynb --to slides --post serve --notes True --local reveal.js` 
but this last one is redundant because if you are using a local reveal.js library the speaker notes are functional by default.

OK, that's all.

Damián.