make tab visible in codemirror and light red background

Shoudl fix #3935

before merging, I would like to find out why there is one line which is suppressed in `IPython/html/static/style/ipython.min.css` 
![capture decran 2013-08-07 a 21 00 51](https://f.cloud.github.com/assets/335567/926840/f08aac9c-ff93-11e2-97b2-3f733fa03e0f.png)