extended SFTPConnectionFactory  for multi-auth username&password PLUS key-file

multi-auth is becoming more common. Large US bank wants username/password PLUS key-file for authentication. Could be done with a service-override (?) and we may need to do that anyway to support client, but this extensioncan be used by other clients as well.

To see this working, edit an OpenSSH configuration file (see http://serverfault.com/a/562899 ) to add this:
AuthenticationMethods publickey,password

behavior in the commandline or scp tool will appear as follows
![uname pwd key](https://cloud.githubusercontent.com/assets/6012630/2806898/d01837de-ccd6-11e3-8e8e-5f95437e8f50.png)