Rudy/psutilcpu

Use Psutil to fix #653 

Keep on using the wmi connection to get the system.cpu.interrupted metric, which is not available via psutil.    I don't know if anyone is really using that metric, given that it's not part of the standard dashboard but I left it there to preserve existing behaviour.

Reason for the >100 peak:
- First, wmi doesn't return values that add to 100 so there was always this offset.
- Second, the `if cpu_user` to check that a value was there resulted in metrics not being sent when they were ==0, which lead to missing values on the graph and aggregation artifacts showing huge peak
Can you take the opportunity to reorganize the import the same way i did ? https://github.com/DataDog/dd-agent/commit/737f396fdd6376232df53db04b96208a675cd0db

Thanks!