css spinner

@gibiansky said: 

>[...] Another user played around with the notebook, then accidentally set off a very long-running computation. He then was very confused as to why no more cells were evaluated – In[*] turns out to be a mediocre way to signal "I'm busy", and he just thought that everything had broken and he had to restart the application and that it was buggy. I think that at least might be easily fixed by letting cells have some sort of spinner indicating business instead of In[*], people are more used to that.

This is my attempt at fixing it with pure css : 

![capture d ecran 2014-12-24 a 14 06 38](https://cloud.githubusercontent.com/assets/335567/5548664/aa1c0942-8b77-11e4-82c8-586548113aab.png)

Smooth fade in/out and height change, rotate while the cell as the `.running` class.

technically I would prefer something like [that](http://css-spinners.com/#/spinner/throbber/) but it require to inject a placeholder for the spinner.

Design is of course to be discussed.