Added a basic WiFi UDP client example (#114)

* Added a basic WiFi UDP client example: a sketch that sends random bytes over udp.

* Updated udp example and included Python and Ruby UDP servers.

libraries/WiFi/examples/WiFiUDPClient/WiFiUDPClient.ino

+/*
+ *  This sketch sends random data over UDP on a ESP32 device
+ *
+ */
+#include <WiFi.h>
+#include <WiFiUdp.h>
+
+// WiFi network name and password:
+const char * networkName = "your-ssid";
+const char * networkPswd = "your-password";
+
+//IP address to send UDP data to:
+// either use the ip address of the server or 
+// a network broadcast address
+const char * udpAddress = "192.168.0.255";
+const int udpPort = 3333;
+
+//Are we currently connected?
+boolean connected = false;
+
+//The udp library class
+WiFiUDP udp;
+
+void setup(){
+  // Initilize hardware serial:
+  Serial.begin(115200);
+  
+  //Connect to the WiFi network
+  connectToWiFi(networkName, networkPswd);
+}
+
+void loop(){
+  //only send data when connected
+  if(connected){
+    //Send a packet
+    udp.beginPacket(udpAddress,udpPort);
+    udp.printf("Seconds since boot: %u", millis()/1000);
+    udp.endPacket();
+  }
+  //Wait for 1 second
+  delay(1000);
+}
+
+void connectToWiFi(const char * ssid, const char * pwd){
+  Serial.println("Connecting to WiFi network: " + String(ssid));
+
+  // delete old config
+  WiFi.disconnect(true);
+  //register event handler
+  WiFi.onEvent(WiFiEvent);
+  
+  //Initiate connection
+  WiFi.begin(ssid, pwd);
+
+  Serial.println("Waiting for WIFI connection...");
+}
+
+//wifi event handler
+void WiFiEvent(WiFiEvent_t event){
+    switch(event) {
+      case SYSTEM_EVENT_STA_GOT_IP:
+          //When connected set 
+          Serial.print("WiFi connected! IP address: ");
+          Serial.println(WiFi.localIP());  
+          //initializes the UDP state
+          //This initializes the transfer buffer
+          udp.begin(WiFi.localIP(),udpPort);
+          connected = true;
+          break;
+      case SYSTEM_EVENT_STA_DISCONNECTED:
+          Serial.println("WiFi lost connection");
+          connected = false;
+          break;
+    }
+}

libraries/WiFi/examples/WiFiUDPClient/udp_server.py

+# This python script listens on UDP port 3333 
+# for messages from the ESP32 board and prints them
+import socket
+import sys
+
+try :
+    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+except socket.error, msg :
+    print 'Failed to create socket. Error Code : ' + str(msg[0]) + ' Message ' + msg[1]
+    sys.exit()
+
+try:
+    s.bind(('', 3333))
+except socket.error , msg:
+    print 'Bind failed. Error: ' + str(msg[0]) + ': ' + msg[1]
+    sys.exit()
+     
+print 'Server listening'
+
+while 1:
+    d = s.recvfrom(1024)
+    data = d[0]
+     
+    if not data: 
+        break
+    
+    print data.strip()
+    
+s.close()
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libraries/WiFi/examples/WiFiUDPClient/udp_server.rb

+# This ruby script listens on UDP port 3333 
+# for messages from the ESP32 board and prints them
+
+require 'socket'
+include Socket::Constants
+
+udp_socket = UDPSocket.new(AF_INET)
+
+#bind 
+udp_socket.bind("", 3333)
+puts 'Server listening'
+
+while true do
+  message, sender = udp_socket.recvfrom(1024)
+  puts message
+end
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