Add missing handlers (#114)

Author: dan12411

lib/client.dart

@@ -24,6 +24,8 @@
 library client;
 
 export 'package:json_api/src/client/client.dart';
+export 'package:json_api/src/client/disposable_handler.dart';
+export 'package:json_api/src/client/persistent_handler.dart';
 export 'package:json_api/src/client/request.dart';
 export 'package:json_api/src/client/response/collection_fetched.dart';
 export 'package:json_api/src/client/response/related_resource_fetched.dart';