Add Udp example from esp8266 forums

Url: http://www.esp8266.com/viewtopic.php?f=29&t=2464&p=14358#p14358

UdpExample/UdpExample.ino

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+/*
+ * 31 mar 2015
+ * This sketch display UDP packets coming from an UDP client.
+ * On a Mac the NC command can be used to send UDP. (nc -u 192.168.1.101 2390). 
+ *
+ * Configuration : Enter the ssid and password of your Wifi AP. Enter the port number your server is listening on.
+ *
+  13 apr 2015. Partly working. Packets are received and displayed, but the confirmation packet is not received by the client.
+  Tested with Mac as client using netcat (nc -u 192.168.1.101 2390)
+
+ */
+#include <ESP8266WiFi.h>
+#include <WiFiUDP.h>
+
+int status = WL_IDLE_STATUS;
+const char* ssid = "yourssid";  //  your network SSID (name)
+const char* pass = "yourpassword";       // your network password
+
+unsigned int localPort = 2390;      // local port to listen for UDP packets
+
+byte packetBuffer[512]; //buffer to hold incoming and outgoing packets
+
+// A UDP instance to let us send and receive packets over UDP
+WiFiUDP Udp;
+
+// Multicast declarations
+IPAddress ipMulti(239, 0, 0, 57);
+unsigned int portMulti = 12345;      // local port to listen on
+
+void setup()
+{
+  // Open serial communications and wait for port to open:
+  Serial.begin(115200);
+
+  // setting up Station AP
+  WiFi.begin(ssid, pass);
+  
+  // Wait for connect to AP
+  Serial.print("[Connecting]");
+  Serial.print(ssid);
+  int tries=0;
+  while (WiFi.status() != WL_CONNECTED) {
+    delay(500);
+    Serial.print(".");
+    tries++;
+    if (tries > 30){
+      break;
+    }
+  }
+  Serial.println();
+
+
+printWifiStatus();
+
+  Serial.println("Connected to wifi");
+  Serial.print("Udp Multicast server started at : ");
+  Serial.print(ipMulti);
+  Serial.print(":");
+  Serial.println(portMulti);
+  Udp.beginMulticast(WiFi.localIP(),  ipMulti, portMulti);
+}
+
+void loop()
+{
+  int noBytes = Udp.parsePacket();
+  if ( noBytes ) {
+    Serial.print(millis() / 1000);
+    Serial.print(":Packet of ");
+    Serial.print(noBytes);
+    Serial.print(" received from ");
+    Serial.print(Udp.remoteIP());
+    Serial.print(":");
+    Serial.println(Udp.remotePort());
+    // We've received a packet, read the data from it
+    Udp.read(packetBuffer,noBytes); // read the packet into the buffer
+
+    // display the packet contents in HEX
+    for (int i=1;i<=noBytes;i++){
+      Serial.print(packetBuffer[i-1],HEX);
+      if (i % 32 == 0){
+        Serial.println();
+      }
+      else Serial.print(' ');
+    } // end for
+    Serial.println();
+    
+    // send a reply, to the IP address and port that sent us the packet we received
+    Udp.beginPacket(Udp.remoteIP(), Udp.remotePort());
+    Udp.write("UDP packet received");
+    Udp.endPacket();    
+  } // end if
+
+
+
+}
+
+void printWifiStatus() {
+  // print the SSID of the network you're attached to:
+  Serial.print("SSID: ");
+  Serial.println(WiFi.SSID());
+
+  // print your WiFi shield's IP address:
+  IPAddress ip = WiFi.localIP();
+  Serial.print("IP Address: ");
+  Serial.println(ip);
+}