added overloads with repoId

Octokit.Reactive/Clients/ObservableMergingClient.cs

@@ -27,5 +27,19 @@ namespace Octokit.Reactive
         {
             return _client.Create(owner, name, merge).ToObservable();
         }
+
+        /// <summary>
+        /// Create a merge for a given repository
+        /// </summary>
+        /// <remarks>
+        /// http://developer.github.com/v3/repos/merging/#perform-a-merge
+        /// </remarks>
+        /// <param name="repositoryId">The ID of the repository</param>
+        /// <param name="merge">The merge to create</param>
+        /// <returns></returns>
+        public IObservable<Merge> Create(int repositoryId, NewMerge merge)
+        {
+            return _client.Create(repositoryId, merge).ToObservable();
+        }
     }
 }