Popover to see sytem resources eaters

I added a popover to see the top 4 of process which are using the sytem resources (RAM and CPU). This list appears when your mouse hover the concerned progress bar (ram or cpu).

For example : 

![image](https://f.cloud.github.com/assets/673926/493112/0ad30d52-bb1e-11e2-88bc-6afb3c99daf9.png)