Add SFTP Support

Certain NASes (Synology) come with an incomplete SSH implementation. They have SFTP, but no SCP.

To work around this, I extended the bamboo-scp-plugin's logic to present an SFTP option. The implementation is admittedly naive at the moment (not a Java guy, sorry), but it was fairly simple to follow the SCP class's example.

Since there was an existing plugin for older Bamboos, I figured there might be some interest in making this a part of the actual plugin. I've included my changes, just in case they're of use to someone.