Fix zmq_socket example in documentation

Issues adressed:
 - The actual data was never read from the socket, causing all even
   numbered loop iterations to fail
 - The socket variable was called server once
Merge pull request #904 from olafmandel/zmq-socket.man

Fix zmq_socket example in documentation
add linking to librt under cmake build - fixes missing symbol

The libzmq library itself uses clock_gettime and should link w/ -lrt on linux.
Caught this issue on on Ubuntu 12.04 LTS.
Merge pull request #905 from guruofquality/missing_librt

add linking to librt under cmake build - fixes missing symbol