[modules/cpu] Add configurable warning and critical thresholds

The cpu module now has cpu.warning and cpu.critical thresholds. If the
CPU utilization is higher than any of those values, the widget's state
changes to warning or critical, respectively.

see #23

bumblebee/output.py

@@ -11,6 +11,7 @@ class Widget(object):
     def __init__(self, full_text="", name=""):
         self._full_text = full_text
         self.module = None
+        self._module = None
         self.name = name
         self.id = str(uuid.uuid4())
 
@@ -20,10 +21,13 @@ class Widget(object):
         This is done outside the constructor to avoid having to
         pass in the module name in every concrete module implementation"""
         self.module = module.name
+        self._module = module
 
     def state(self):
         """Return the widget's state"""
-        return "state-default"
+        if self._module and hasattr(self._module, "state"):
+            return self._module.state(self)
+        return None
 
     def full_text(self):
         """Retrieve the full text to display in the widget"""