adding ethernet example

It took more than a couple minutes to get some sample code going with ethernet, so I figured it would benefit everyone if there was at least one example out there.

examples/mqtt_ethernet/mqtt_ethernet.ino

@@ -0,0 +1,144 @@
+/***************************************************
+  Adafruit MQTT Library Ethernet Example
+
+  Adafruit invests time and resources providing this open source code,
+  please support Adafruit and open-source hardware by purchasing
+  products from Adafruit!
+
+  Written by Alec Moore
+  Derived from the code written by Limor Fried/Ladyada for Adafruit Industries.
+  MIT license, all text above must be included in any redistribution
+ ****************************************************/
+#include <SPI.h>
+#include "Adafruit_MQTT.h"
+#include "Adafruit_MQTT_Client.h"
+
+#include <Ethernet.h>
+#include <EthernetClient.h>
+#include <Dns.h>
+#include <Dhcp.h>
+
+/************************* Ethernet Client Setup *****************************/
+byte mac[] = {0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED};
+
+//Uncomment the following, and set to a valid ip if you don't have dhcp available.
+//IPAddress iotIP (192, 168, 0, 42);
+//Uncomment the following, and set to your preference if you don't have automatic dns.
+//IPAddress dnsIP (8, 8, 8, 8);
+//If you uncommented either of the above lines, make sure to change "Ethernet.begin(mac)" to "Ethernet.begin(mac, iotIP)" or "Ethernet.begin(mac, iotIP, dnsIP)"
+
+
+/************************* Adafruit.io Setup *********************************/
+
+#define AIO_SERVER      "io.adafruit.com"
+#define AIO_SERVERPORT  1883
+#define AIO_USERNAME    "tapiralec"
+#define AIO_KEY         "f2983986545d28bdad80ba706b01bc1725417aab"
+
+/************ Global State (you don't need to change this!) ******************/
+
+//Set up the ethernet client
+EthernetClient client;
+
+// Store the MQTT server, client ID, username, and password in flash memory.
+// This is required for using the Adafruit MQTT library.
+const char MQTT_SERVER[] PROGMEM    = AIO_SERVER;
+// Set a unique MQTT client ID using the AIO key + the date and time the sketch
+// was compiled (so this should be unique across multiple devices for a user,
+// alternatively you can manually set this to a GUID or other random value).
+const char MQTT_CLIENTID[] PROGMEM  = __TIME__ AIO_USERNAME;
+const char MQTT_USERNAME[] PROGMEM  = AIO_USERNAME;
+const char MQTT_PASSWORD[] PROGMEM  = AIO_KEY;
+
+Adafruit_MQTT_Client mqtt(&client, MQTT_SERVER, AIO_SERVERPORT, MQTT_CLIENTID, MQTT_USERNAME, MQTT_PASSWORD);
+
+// You don't need to change anything below this line!
+#define halt(s) { Serial.println(F( s )); while(1);  }
+
+
+/****************************** Feeds ***************************************/
+
+// Setup a feed called 'photocell' for publishing.
+// Notice MQTT paths for AIO follow the form: <username>/feeds/<feedname>
+const char PHOTOCELL_FEED[] PROGMEM = AIO_USERNAME "/feeds/photocell";
+Adafruit_MQTT_Publish photocell = Adafruit_MQTT_Publish(&mqtt, PHOTOCELL_FEED);
+
+// Setup a feed called 'onoff' for subscribing to changes.
+const char ONOFF_FEED[] PROGMEM = AIO_USERNAME "/feeds/onoff";
+Adafruit_MQTT_Subscribe onoffbutton = Adafruit_MQTT_Subscribe(&mqtt, ONOFF_FEED);
+
+/*************************** Sketch Code ************************************/
+
+void setup() {
+  Serial.begin(115200);
+
+  Serial.println(F("Adafruit MQTT demo"));
+
+  Serial.print(F("Free RAM: ")); Serial.println(getFreeRam(), DEC);
+
+  // Initialise the Client
+  Serial.print(F("\nInit the Client..."));
+  Ethernet.begin(mac);
+  delay(1000); //give the ethernet a second to initialize
+  
+
+  mqtt.subscribe(&onoffbutton);
+
+  }
+}
+
+uint32_t x=0;
+
+void loop() {
+  // Ensure the connection to the MQTT server is alive (this will make the first
+  // connection and automatically reconnect when disconnected).  See the MQTT_connect
+  // function definition further below.
+  MQTT_connect();
+ 
+  // Try to ping the MQTT server
+  if (! mqtt.ping(3) ) {
+    // MQTT pings failed, let's reconnect by forcing a watchdog reset.
+    Serial.println("Ping fail! Resetting...");
+    delay(10000);
+  }
+
+  // this is our 'wait for incoming subscription packets' busy subloop
+  Adafruit_MQTT_Subscribe *subscription;
+  while ((subscription = mqtt.readSubscription(1000))) {
+    if (subscription == &onoffbutton) {
+      Serial.print(F("Got: "));
+      Serial.println((char *)onoffbutton.lastread);
+    }
+  }
+
+  // Now we can publish stuff!
+  Serial.print(F("\nSending photocell val "));
+  Serial.print(x);
+  Serial.print("...");
+  if (! photocell.publish(x++)) {
+    Serial.println(F("Failed"));
+  } else {
+    Serial.println(F("OK!"));
+  }
+}
+
+// Function to connect and reconnect as necessary to the MQTT server.
+// Should be called in the loop function and it will take care if connecting.
+void MQTT_connect() {
+  int8_t ret;
+
+  // Stop if already connected.
+  if (mqtt.connected()) {
+    return;
+  }
+
+  Serial.print("Connecting to MQTT... ");
+
+  while ((ret = mqtt.connect()) != 0) { // connect will return 0 for connected
+       Serial.println(mqtt.connectErrorString(ret));
+       Serial.println("Retrying MQTT connection in 5 seconds...");
+       mqtt.disconnect();
+       delay(5000);  // wait 5 seconds
+  }
+  Serial.println("MQTT Connected!");
+}