ADD: Get the response code from a failing connection.

This commit adds an event information "connect_return_code" that is written
when the client state is MQTT_STATE_INIT, and the connection fails.

This event info can then be written by the user app to find out the reason
of the fail connection.

Merges https://github.com/espressif/esp-mqtt/pull/100

lib/include/mqtt_msg.h

@@ -57,15 +57,6 @@ enum mqtt_message_type {
     MQTT_MSG_TYPE_DISCONNECT = 14
 };
 
-enum mqtt_connect_return_code {
-    CONNECTION_ACCEPTED = 0,
-    CONNECTION_REFUSE_PROTOCOL,
-    CONNECTION_REFUSE_ID_REJECTED,
-    CONNECTION_REFUSE_SERVER_UNAVAILABLE,
-    CONNECTION_REFUSE_BAD_USERNAME,
-    CONNECTION_REFUSE_NOT_AUTHORIZED
-};
-
 typedef struct mqtt_message {
     uint8_t *data;
     uint32_t length;