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/ObservableGitHubClient.cs

@@ -46,6 +46,7 @@ namespace Octokit.Reactive
             Gist = new ObservableGistsClient(gitHubClient);
             Search = new ObservableSearchClient(gitHubClient);
             Enterprise = new ObservableEnterpriseClient(gitHubClient);
+            Migration = new ObservableMigrationClient(gitHubClient);
         }
 
         public IConnection Connection
@@ -72,6 +73,7 @@ namespace Octokit.Reactive
         public IObservableGitDatabaseClient Git { get; private set; }
         public IObservableSearchClient Search { get; private set; }
         public IObservableEnterpriseClient Enterprise { get; private set; }
+        public IObservableMigrationClient Migration { get; private set; }
 
         /// <summary>
         /// Gets the latest API Info - this will be null if no API calls have been made