allow to set rpc.NamenodeConnection for the hdfs client

[advincze]

in my case I use a Socks proxy to access hdfs. I can already use the rpc.WrapNamenodeConnection to create an *rpc.NamenodeConnection but I cannot use it for the client.

This would work:

func NewForNamenode(namenode *rpc.NamenodeConnection) *Client {
	return &Client{namenode: namenode}
}

If if fits the project should I submit a PR ?

[colinmarc]

This is a good idea - I'd call it NewForConnection though.