add a dirty class to cell when modified

Allow to do stuff like : 
![capture decran 2013-10-08 a 17 21 28](https://f.cloud.github.com/assets/335567/1290212/08d4331c-302d-11e3-84af-1c70ac5c5181.png)

with this css
```
<style>
.dirty div.input div.prompt{
    color:grey;
}

.dirty div.prompt{
    color:grey;
}
</style>
```
All cells that have not been executed in the current page have grey prompt, prompt get colored if you execute the cell. It revert back to grey if you ever trigger a 'dirty' on code mirror.