Refactor: Pull out Migrations client from Enterprise.

Following the API sidebar structure, the migrations client is pulled out
and put into another Migration client.

Octokit.Reactive/Clients/ObservableMigrationClient.cs

@@ -0,0 +1,20 @@
+namespace Octokit.Reactive
+{
+    public class ObservableMigrationClient : IObservableMigrationClient
+    {
+        public ObservableMigrationClient(IGitHubClient client)
+        {
+            Ensure.ArgumentNotNull(client, "client");
+
+            Migration = new ObservableMigrationsClient(client);
+        }
+
+        /// <summary>
+        /// A client for GitHub's Enterprise Migrations API.
+        /// </summary>
+        /// <remarks>
+        /// https://developer.github.com/v3/migration/#enterprise-migrations
+        /// </remarks>
+        public IObservableMigrationsClient Migration { get; private set; }
+    }
+}