reST cell type (render in server-side as a whole document) 

This is my first attempt to implement #3714 , and although it seems to be working well, take it as a proof of concept to get early feedback. 

I know this is recurrent and controversial feature (#888, 1331 and may be more) but paraphasing somebody in an issue comment (I've read it yesterday but can't find the source now) who said: *"In IPython we code what we need"* . Of course, I (and hopefully more people) need it :)

I've could also argue that reStructuredText is the "de facto" markup to write "documents" in a "pythonic way" being a markup so much powerful than markdown. Having support in the notebook would mean a kind of powerful front-end editor for tools like Sphinx, Nikola, etc. 

## The approach

The idea is very simple: join all rst cells as a whole document adding a separation mark, render via docutils (in server side), split the html into cell "chunks"  (using those marks), and inject each chunk in the correspondent cell. 

This simple idea solves the problem [described by @minrk](https://github.com/ipython/ipython/issues/3714#issuecomment-21313844) about rendering rST one cell at a time . 

I would like to read your opinions

PS: sorry if my english isn't so clear.