Nbviewer2 theme

On top of 40. 

Allow to ask for a specific "theme" in the notebook metadata. 

By editting your cookies you also "force" a theme of any notebook. 

I added 2 css the one of [this](http://gistpynb.herokuapp.com/urls/raw.github.com/CamDavidsonPilon/Probabilistic-Programming-and-Bayesian-Methods-for-Hackers/master/Chapter1_Introduction/Chapter1_Introduction.ipynb) notebook, and [this one](http://nbviewer.ipython.org/urls/raw.github.com/carljv/cython_testing/master/cython_linalg.ipynb)