Introduce xvfb support for Selenium tests

Now when 'gradle selenium' is run, the Web application will be loaded into a Jetty server and served on port 0.0.0.0:8080/chuckanator and Xvfb will be started on display :99 so that Firefox can be run in headless mode. Then the selenium tests are run.

Note: sometimes the Java process is not ended.
Fix installApp and run Gradle tasks

Missing main function for the Class was missing, so installApp and run tasks couldn't be run.