Support for SSH agent forwarding

I am currently adding support for SSH agent forwarding in phpseclib with the goal of providing SSH agent forwarding support in rocketeer. As this library is a dependency of rocketeer I need to add support for agent forwarding here as well. This is my first run at it. Comments and feedback please. 

See https://github.com/phpseclib/phpseclib/pull/518 for agent forwarding support in phpseclib.