Added method ZMQ.Socket.setProbeRouter(), issue #333

This seems pretty straightforward, but it's my first attempt at contributing, so please be gentle. I've written a simple test, along the lines of test_probe_router.cpp in the core library tests, and it appears to work properly. Should I incorporate it in the junit tests, and if so, which file would be the appropriate place to put it?

Also, I tried writing a getProbeSocket method, but it ends up that zmq_getsockopt in the core library doesn't support the ZMQ_PROBE_ROUTER option. I don't know whether that constitutes an issue or not. It might be intentional; the documentation for zmq_getsockopt doesn't mention ZMQ_PROBE_ROUTER.