Start of SSH2 Agent Forwarding implementation

As I am rather keen on having this work I've written code to support a SSH2 Agent Forwarding implementation. This PR does enable this functionality but as I am not super familiar with PHP or the design of this library I'm expecting some feedback or comments. 

This PR will address this current OPEN issue / feature request. 

https://github.com/phpseclib/phpseclib/issues/505
Yes I can see now we are using EOF channel state to track state about who initiated CLOSE. This only works because EOF message from server is ignored (with regards to channel state transition). In that case I probably broke this by starting to track server initiated EOF. Would you prefer to implement it this way rather than track if client CLOSE has been sent explicitly?I don't know if server initiated EOF is actually useful practically - but it would reflect the actual state of the channel.