java: implement getClasspath() method in system class loader

Tomcat tries to invoke that method on the class loader if it is not instance
of URLClassLoader.

Makes the messages like this go away:

 Jan 22, 2014 12:30:21 PM org.apache.catalina.loader.WebappLoader buildClassPath
 INFO: Unknown loader io.osv.OsvSystemClassLoader@fce56f8 class io.osv.OsvSystemClassLoader