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/project.json

@@ -0,0 +1,13 @@
+{
+  "version": "1.0.0-*",
+
+  "dependencies": {
+    "NETStandard.Library": "1.5.0-rc2-24027"
+  },
+
+  "frameworks": {
+    "netstandard1.5": {
+      "imports": "dnxcore50"
+    }
+  }
+}