Add observer pattern

Update memento pattern examples class to have run as static method
Update behavioral patterns program.cs to maintain the order

src/ObserverPattern/Twits/TwitUser.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace ObserverPattern.Twits
+{
+    /// <summary>
+    /// Concrete observer
+    /// </summary>
+    public class TwitUser : IObserver<string>
+    {
+        string name;
+        public TwitUser(string name)
+        {
+            this.name = name;
+        }
+        public void OnCompleted()
+        {
+            Console.WriteLine("{0} finished watching twitter", name);
+        }
+
+        public void OnError(Exception error)
+        {
+            Console.WriteLine("Error while watching twitter: {0}", error);
+        }
+
+        public void OnNext(string value)
+        {
+            Console.WriteLine("{0} just observed that something {1} was tweeted", name, value);
+        }
+    }
+}