* Adds {@link TwillRunnable} TwillRunnable as a Custom Service {@link TwillApplication} to the Application.
* @param runnable TwillRunnable to run as service
* @param specification ResourceSpecification for Twill container.
*/
private void addService(TwillRunnable runnable, ResourceSpecification specification) {
addService(new SingleRunnableApplication(runnable, specification));
}