make url property of FileUploaderOptions optional

libs/ng2-file-upload/file-upload/file-uploader.class.ts

@@ -31,7 +31,7 @@ export interface FileUploaderOptions {
   maxFileSize?: number;
   queueLimit?: number;
   removeAfterUpload?: boolean;
-  url: string;
+  url?: string;
   disableMultipart?: boolean;
   itemAlias?: string;
   authTokenHeader?: string;