fix(typings): Correct typings for StageComponent

Turned StageComponent into a class with static functions as TypeScript would turn the JSON const to `any`.
Fixes aurelia#46

src/component-tester.js

@@ -1,11 +1,11 @@
 import {View} from 'aurelia-templating';
 import {Aurelia} from 'aurelia-framework';
 
-export const StageComponent = {
-  withResources(resources): ComponentTester {
+export class StageComponent {
+  static withResources(resources: string | string[]): ComponentTester {
     return new ComponentTester().withResources(resources);
   }
-};
+}
 
 export class ComponentTester {
   bind: (bindingContext: any) => void;