Socket option getters

This is a response to @alecthomas' comment on #15, "I haven't merged this mainly because I haven't updated it for the new tagged version builds."  I'm not sure this code is ready for the master branch yet, but both `go test` (with libzmq-2.2.0) and `go test -tags zmq_3_x` (with libzmq-3.2.2) build and pass.