Create BodyParser and refakt Rest package

modified:   lib/resource.dart
modified:   lib/rest.dart
modified:   lib/src/resource/resource_event.dart
new file:   lib/src/rest/body_parser.dart
modified:   lib/src/rest/rest_request.dart
modified:   lib/src/rest/rest_response.dart
modified:   test/rest_test.dart