* Add mysensors binary sensor.
* Add discovery platforms to binary_sensor base component.
* Replace device_state_attributes with state_attributes in
binary_sensor base class.
* Fix docstrings.
* Add discovery of binary sensor to mysensors component.
* Add child.type as argument to mysensors device_class.
* Move binary sensor types from sensor to binary_sensor module.
* Fix binary_sensor attribute tests. Use state_attributes instead of
device_state_attributes.
This adds a 'sensor_class' property and attribute, which should be either
None or one of several defined SENSOR_CLASSES to indicate contextual
information about what the sensor is measuring.
