[modules/cpu] Add initial version of CPU utilization module

Re-enable the CPU utilization module as proof-of-concept for the new
core engine.

see #23

bumblebee/modules/test.py

@@ -9,6 +9,6 @@ class Module(bumblebee.engine.Module):
         super(Module, self).__init__(engine)
 
     def widgets(self):
-        return []
+        return bumblebee.output.Widget(full_text="test")
 
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