[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

tests/util.py

@@ -31,9 +31,17 @@ class MockEngine(object):
         self.input = MockInput()
 
 class MockConfig(object):
+    def __init__(self):
+        self._data = {}
+
     def get(self, name, default):
+        if name in self._data:
+            return self._data[name]
         return default
 
+    def set(self, name, value):
+        self._data[name] = value
+
 class MockOutput(object):
     def start(self):
         pass