DO NOT MERGE YET! added BlockWriterDialTimeout to namenode

[advincze]

see #70 for the problem statement

this is how i use it with a sock5 proxy

	socksProxy := "localhost:8157"
	var dialerFn = func(network, address string, timeout time.Duration) (net.Conn, error) {
		if socksProxy != "" {
			dialer, _ := proxy.SOCKS5("tcp", socksProxy, nil, proxy.Direct)
			return dialer.Dial(network, address)
		}
		return net.DialTimeout(network, address, timeout)
	}

	c, _ := dialerFn("tcp", nameNodeAddr, time.Second)
	nc, _ := rpc.WrapNamenodeConnection(c, hdfsUser)
	nc.BlockWriterDialTimeout = dialerFn
	hdfsCli := hdfs.NewForConnection(nc)
	defer hdfsCli.Close()