adds packet ids for publish QoS > 0

also adds packet id counter so packet ids aren't
static values. this is a step in the direction
of QoS 1 support

Adafruit_MQTT.h

@@ -193,6 +193,7 @@ class Adafruit_MQTT {
   uint8_t will_qos;
   uint8_t will_retain;
   uint8_t buffer[MAXBUFFERSIZE];  // one buffer, used for all incoming/outgoing
+  uint16_t packet_id_counter = 0;
 
  private:
   Adafruit_MQTT_Subscribe *subscriptions[MAXSUBSCRIPTIONS];