Add mocha test bundling with webpack and test running

tests/javascript/newsletter_editor/testAjax.js

@@ -0,0 +1,7 @@
+define('testAjax', [ 'mailpoet', 'ajax'], function(MailPoet) {
+  describe('Ajax submodule', function() {
+    it('has a version', function() {
+      expect(MailPoet.Ajax.version).to.be.a('number');
+    });
+  });
+});