Create observable commits client

Octokit.Reactive/Clients/ObservableGitDatabaseClient.cs

@@ -5,8 +5,10 @@
         public ObservableGitDatabaseClient(IGitHubClient client)
         {
             Tag = new ObservableTagsClient(client);
+            Commit = new ObservableCommitsClient(client);
         }
 
         public IObservableTagsClient Tag { get; set; }
+        public IObservableCommitsClient Commit { get; set; }
     }
 }