Added Observable classes and got started on observable tests

Octokit.Tests/Reactive/ObservableUserAdministrationClientTests.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using NSubstitute;
+using Octokit.Reactive;
+using Xunit;
+using System.Reactive.Linq;
+
+namespace Octokit.Tests.Reactive
+{
+    public class ObservableUserAdministrationClientTests
+    {
+        public class ThePromoteMethod
+        {
+            [Fact]
+            public void EnsuresArgumentIsNotNull()
+            {
+                var client = new ObservableUserAdministrationClient();                
+                Assert.Throws<ArgumentNullException>(() =>  client.Promote(null));
+            }
+        }
+    }
+}