Update example platforms for async conventions (#1998)

source/developers/platform_example_sensor.markdown

@@ -38,6 +38,10 @@ def setup_platform(hass, config, add_devices, discovery_info=None):
 class ExampleSensor(Entity):
     """Representation of a Sensor."""
 
+    def __init__(self):
+        """Initialize the sensor."""
+        self._state = None
+
     @property
     def name(self):
         """Return the name of the sensor."""
@@ -46,10 +50,17 @@ class ExampleSensor(Entity):
     @property
     def state(self):
         """Return the state of the sensor."""
-        return 23
+        return self._state
 
     @property
     def unit_of_measurement(self):
         """Return the unit of measurement."""
         return TEMP_CELSIUS
+
+    def update(self):
+        """Fetch new state data for the sensor.
+
+        This is the only method that should fetch new data for Home Assistant.
+        """
+        self._state = 23
 ```