Added support for tellstick devices and sensors

Adds the functionality to track and control tellstick devices and sensors
Adding the unit to the state directly will reduce re-usability by other components.

So I've added a new attribute that you can add to the state: [homeassistant.components.ATTR_UNIT_OF_MEASUREMENT](https://github.com/balloob/home-assistant/blob/master/homeassistant/components/__init__.py#L35). If you add this as state attribute, it will be displayed in the frontend: `{"unit_of_measurement": "°C"}`