Customising the UI


This is a pull request for [this issue](https://github.com/ipython/ipython/issues/5551)

In my opinion the user interface could be improved. On a 13" screen the headers and toolbars take up too much of the vertical real estate. With that in mind, I spent a few hours redesigning my own IPython. My fork is up on [github](http://github.com/eoinmurray/ipython). 

I would have preferred to only change my own ```custom.css``` in my IPython profile, but a fair amount of the notebooks view elements are contained in the javascript. So a fork seemed the best option. 

Could there be some method for allowing users to customise the UI more with just the ```custom.css``` and ```custom.js```? Or perhaps some way that custom ```IPython/html/static``` and ```IPython/html/templates``` can be installed/uninstalled.

I would like to work on detaching the UI from the javascript if there is some interest in this happening. underscore.js templates might be an option for all the UI elements that the IPython notebook dynamically inserts into the page. This would mean people would not need to touch the javascript when customising.

![screenshot 2014-04-06 23 31 42](https://cloud.githubusercontent.com/assets/1448368/2635959/67bb07a6-be8b-11e3-8c41-552d71fb9aea.png)

if you need a change like that, I would introduce a class `.ipython-loginwidget` and have

```
.ipython-loginwidget {
    .btn();
    .btn-xs();
}
```

in theme for easy overwrite.