Add ability to monitor/manage the Jetty instance through JMX

The fact that you cannot connect anymore to our standalone distribution using jconsole may be a regression caused by our upgrade to Jetty 9 (see XWIKI-3622) since I'm pretty sure it used to work fine before. It seems that with Jetty 9 you need to pass "-Dcom.sun.management.jmxremote" on the command line now.

In addition Jetty has some MBeans to perform monitoring and management of the Jetty server itself. We should add them too since they can prove useful.