Add getsockopt method

Corresponds to nn_getsockopt. Retrives the socket option for the
specified (level, option) tuple.

Example usage:

```
        let addr="tcp://127.0.0.1:8898";

        let mut sock = match NanoSocket::new(AF_SP, NN_PAIR) {
            Ok(s) => s,
            Err(_) => fail!("asdf")
        };

        sock.bind(addr);

        // Linger default is 1000ms
        let ret = match sock.getsockopt(NN_SOL_SOCKET, NN_LINGER) {
          Ok(s) => s,
          Err(e) => fail!("failed getsockopt(NN_SOL_SOCKET, NN_LINGER) with err:{:?} {:?}", e.rc, e.errstr)
        };

        assert!(ret == 1000);
```

More examples in the added tests.