Added loading indicator support to the ajax component

Added the option to display a loading indicator while requesting a component asynchronously. Utilizes spin.js to generate the spinner. Showing the loading indicator can be enabled/disabled per component by setting a property "showLoading" to {Boolean}true. Per default, the loading indicator is not shown.

![screenshot 2014-06-15 17 14 07](https://cloud.githubusercontent.com/assets/7879919/3281287/c4429b16-f49f-11e3-99a0-c1627ba8e87d.png)