Handle heartbeat message

[MartinHjelmare]

• Add method _handle_heartbeat to handle internal message of type
  I_HEARTBEAT_RESPONSE.
• Add an instance attribute new_state to Sensor class. This attribute
  represents the new state of the dictionary stored in Sensor
  attribute children. The new state is what a controller wants to
  achieve. The children dictionary is only
  updated when I_HEARTBEAT_RESPONSE is received and _handle_heartbeat
  is called.
• Add attribute queue of type collections.deque in Sensor class. Use
  the deque to store replies for received messages. Add replies to
  queue when heartbeat is received.
• Fill Gateway.queue in _handle_heartbeat with commands from new_state
  and Sensor.queue. Get tasks from Gateway.queue until it's
  empty in run loop before receiving new messages.
• Update set_child_value method in Sensor class to handle kwarg
  children. This is used in Gateway method set_child_value to update
  values of new_state Sensor attribute.
• Update method _handle_internal to redirect message with
  I_HEARTBEAT_RESPONSE to _handle_heartbeat.
• Update set_child_value in Gateway class, to not fill queue and
  instead update new_state dict if new_state is non empty. It's non
  empty if _handle_heartbeat has been called at least once.
• Clean up send method in SerialGateway class.
• Update run methods in SerialGateway and TCPGateway where send method
  is called.
• Add str and repr magic methods to ChildSensor class.
• Add tests for heartbeat message.
• Update tests to handle updated logic method return type.
• Check protocol version before checking heartbeat internal message.
• Remove unnecessary log message.
• Remove unnecessary return statements.
• Add validation of child values.
• Fix check of protocol version at Gateway init.

[MartinHjelmare]

Needs testing before merging.