Add support for StatusEventPayload (#1732)

* added support for StatusEventPayload

* Added test for StatusEventPayload and fixed serializer to return that event payload type.

Octokit/Http/SimpleJsonSerializer.cs

@@ -225,6 +225,8 @@ namespace Octokit.Internal
                         return typeof(PullRequestCommentPayload);
                     case "PushEvent":
                         return typeof(PushEventPayload);
+                    case "StatusEvent":
+                        return typeof(StatusEventPayload);
                     case "WatchEvent":
                         return typeof(StarredEventPayload);
                 }