More css

Some cleanup, of extensions. 
add .gitignore for custom.js and custom.css (so that you can pull newer version without changing your conf)


Pushed by @ellisonbg ipython/ipython#3631 I added an XKCD theme to the theme selector.

![capture decran 2013-07-19 a 15 58 33](https://f.cloud.github.com/assets/335567/826014/69b01e5e-f07b-11e2-8fae-3d58d948f41e.png)
![capture decran 2013-07-19 a 15 58 45](https://f.cloud.github.com/assets/335567/826013/699b8782-f07b-11e2-8f64-767f48eee695.png)