merged with css/html replacements from notebook_converter.py from jakevdp

Hi,

this is my first ever pull request, so please excuse potential mistakes.
I tried to add notebooks to my pelican side and found that the ipython styles were lost, as noted here:
http://jakevdp.github.io/blog/2012/10/04/blogging-with-ipython/

Jake Vanderplas made modifications to the generated header css via a script. I could not figure out how to run this, so I moved these css modifications in here, right next to the replacement of .highlight with .highlight-ipynb which was already present. 

Seems to me, these modifications should better be bundled in one place.