A new implementation of reveal converter with jinja templates.

OK, this is my implementation of reveal converter using jinja templates.

It uses the power of jinja template inheritance and a more concise and shorter code than @Carreau 's implementation in transformers.py. 
The logic to get slides and subslides is very different: using the fact of some details of the markup structure in reveal library is easier now to delimit slides and subslides with nested <section> and </section> provided by the proposed templating structure (no more need for all the helper code)

All the relevant information and discussion about the development of this converter lives in this [PR#69]( https://github.com/ipython/nbconvert/pull/69).

**NOTE: This converter NEEDS some code derived from the original implementation for the reveal converter [PR#69](https://github.com/ipython/nbconvert/pull/69), so it is necessary to finish with that PR#69) (merged or rejected... in the last case I would have to migrate all the remaining code) to get a functional slideshow.**

ping @Carreau @ellisonbg