HASensor improvements (#222)

* added support for setting None value in the HASensor

* added support for the expire_after property in the HASensor

* update readme and docs

* added expire_after property in the HABinarySensor

* increase default device types limit to 24 on non ATMega328/168 processors

* added support for the JSON attributes in the HASensor

examples/sensor-integer/sensor-integer.ino

@@ -37,5 +37,8 @@ void loop() {
         unsigned long uptimeValue = millis() / 1000;
         uptimeSensor.setValue(uptimeValue);
         lastUpdateAt = millis();
+
+        // you can reset the sensor as follows:
+        // analogSensor.setValue(nullptr);
     }
 }