Add missing plain and clone method to restangular IElement

See https://github.com/mgonto/restangular#element-methods for documentation on the methods.

restangular/restangular.d.ts

@@ -112,6 +112,8 @@ declare module restangular {
     trace(queryParams?: any, headers?: any): IPromise<any>;
     options(queryParams?: any, headers?: any): IPromise<any>;
     patch(queryParams?: any, headers?: any): IPromise<any>;
+    clone(): IElement;
+    plain(): any;
     withHttpConfig(httpConfig: IRequestConfig): IElement;
     getRestangularUrl(): string;
   }