Jenkins CLI now connect to existing SSH agent for authentication

If an existing SSH agent is detected, the CLI will try to use its known identities to authenticate towards Jenkins. This allow the CLI to not ask the SSH key passphrase at each command.

Agent communication is done through project jsch-agent-proxy (https://github.com/ymnk/jsch-agent-proxy), this new dependency increase the jar size from 2.5 to 3.9 mb.

This has been successfully tested both on OSX (with the OpenSSH agent) and on Windows 8 with Pageant.