A special ClassLoader to load the Jetty 8 dependency of PhantomJSEnv
in a private space.
It loads everything that belongs to JettyWebsocketManager itself (while
retrieving the requested class file from its parent.
For all other classes, it first tries to load them from jettyLoader,
which should only contain the Jetty 8 classpath.
If this fails, it delegates to its parent.
The rationale is, that JettyWebsocketManager and its dependees can use
the classes on the Jetty 8 classpath, while they remain hidden from the rest
of the Java world. This allows to load another version of Jetty in the same
JVM for the rest of the project.