Correct typo
Add sftp.NewClientPipe()

This method returns a Client from a pair of pipes.  This can be used for
connecting to an SFTP server over TCP/TLS or by using the system's ssh
client program (e.g. via exec.Command).

An Example function is added, and the client integration test uses the
function.
Merge pull request #21 from fd0/client-from-io

Add sftp.NewClientPipe