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/ObservableTwitsExample.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace ObserverPattern.Twits
+{
+    public class ObservableTwitsExample
+    {
+        public void Run()
+        {
+            TwitObservable observable = new TwitObservable();
+
+            TwitUser t100 = new TwitUser("t100");
+            TwitUser r2d2 = new TwitUser("r2d2");
+            
+            var t100Subscription = observable.Subscribe(t100);
+            var r2d2Subscription = observable.Subscribe(r2d2);
+
+            observable.AddTwit("El chupacapra");
+
+            t100Subscription.Dispose();
+
+            observable.AddTwit("Vamos vamos mi amor");
+
+            observable.ItsGoingHomeTime();
+
+            
+        }
+    }
+}