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/ObserverPatternExamples.cs

@@ -0,0 +1,18 @@
+using ObserverPattern.Twits;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace ObserverPattern
+{
+    
+    public class ObserverPatternExamples
+    {
+        public static void Run()
+        {
+            ObservableTwitsExample obsTwits = new ObservableTwitsExample();
+            obsTwits.Run();
+        }
+    }
+}