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/IObservableMigrationClient.cs

@@ -0,0 +1,13 @@
+namespace Octokit.Reactive
+{
+    public interface IObservableMigrationClient
+    {
+        /// <summary>
+        /// A client for GitHub's Migrations API
+        /// </summary>
+        /// <remarks>
+        /// See the <a href="https://developer.github.com/v3/migration/migrations/">Enterprise License API documentation</a> for more information.
+        /// </remarks>
+        IObservableMigrationsClient Migration { get; }
+    }
+}