Add files for task 3

Tasks/UrlTester/DT3/DT3cs.cs

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+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Diagnostics;
+using System.Threading.Tasks;
+using System.Windows;
+using DT2;
+
+namespace DT3 {
+
+    public class UrlTesterModel /* TO BE DONE */ {
+        public event PropertyChangedEventHandler PropertyChanged;
+
+        int size;
+        int time;
+        string url;
+        DT2.UrlTester urlTester;
+
+        public UrlTesterModel() {
+            urlTester = new DT2.UrlTester();
+            IDictionary<string, Stopwatch> sw = new Dictionary<string, Stopwatch>();
+            urlTester.PageStart += url => { sw.Add(url, Stopwatch.StartNew()); };
+            urlTester.PageLoaded += (url, size) => {
+                sw[url].Stop();
+                int time = (int)sw[url].Elapsed.TotalMilliseconds;
+                Size = size;
+                Time = time;
+                sw.Remove(url);
+            };
+        }
+
+        public int Size {
+            get { return size; }
+            set /* TO BE DONE */
+        }
+
+        public int Time {
+            get { return time; }
+            set /* TO BE DONE */
+        }
+
+        public string Url {
+            get { return url; }
+            set /* TO BE DONE */
+        }
+    }
+
+}