allow specify classes/packages to be loaded by system class loader

Pull request that provides an option to instruct the launcher to load certain classes using system class loader to avoid ClassCastException when running in hosted mode. List of classes/packages can be given as system property called: jetty.custom.sys.classes as semicolon separated list

This resolves issue when using bitronix transaction manager (version 3.x) in hosted mode that is configured as resources in jetty-env.xml (used in jbpm)

Pull request is targeting 2.3 branch as this is what jbpm (and drools) are using at the moment.