implementation of I_DISCOVER

Discovery function for all active (=listening) nodes. Nodes reply with
I_DISCOVER_RESPONSE and the parent node as payload. Useful to retrieve
network topology.

libraries/MySensors/core/MyMessage.h

@@ -152,7 +152,7 @@ typedef enum {
 	I_INCLUSION_MODE, I_CONFIG, I_FIND_PARENT, I_FIND_PARENT_RESPONSE,
 	I_LOG_MESSAGE, I_CHILDREN, I_SKETCH_NAME, I_SKETCH_VERSION,
 	I_REBOOT, I_GATEWAY_READY, I_REQUEST_SIGNING, I_GET_NONCE, I_GET_NONCE_RESPONSE,
-	I_HEARTBEAT, I_PRESENTATION
+	I_HEARTBEAT, I_PRESENTATION, I_DISCOVER, I_DISCOVER_RESPONSE
 } mysensor_internal;