added proper reactive client

Octokit.Reactive/Clients/ObservableRepositoryCommitsClients.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Reactive.Threading.Tasks;
+
+namespace Octokit.Reactive
+{
+    public class ObservableRepositoryCommitsClient : IObservableRepositoryCommitsClient
+    {
+        readonly IGitHubClient _client;
+
+        public ObservableRepositoryCommitsClient(IGitHubClient client)
+        {
+            _client = client;
+        }
+
+        /// <summary>
+        /// Compare two references in a repository
+        /// </summary>
+        /// <param name="owner">The owner of the repository</param>
+        /// <param name="name">The name of the repository</param>
+        /// <param name="base">The reference to use as the base commit</param>
+        /// <param name="head">The reference to use as the head commit</param>
+        /// <returns></returns>
+        public IObservable<CompareResult> Compare(string owner, string name, string @base, string head)
+        {
+            return _client.Repository.Commits.Compare(owner, name, @base, head).ToObservable();
+        }
+    }
+}