[IPython-dev] improve http://nbviewer.ipython.org/

[Carl Smith]

Matthias, just an aside: nbconvert puts ugly red borders round certain
chars in code cells. I've not had a chance to look at this properly,
but dropping the following CSS at the bottom of the head seems to fix
things for now.

.highlight .err { border: none; }

I also added `color: red;` to this rule to add some highlighting, but
this is just a cheap and inconsistent fix until I can look at it.
Should be a five minute job, but I've not got round to it.