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
NOTE: I changed this because according to rfc4254  5.3 "... Upon receiving this message, a party MUST send back an SSH_MSG_CHANNEL_CLOSE" in conjunction with " ... A party MAY send
   SSH_MSG_CHANNEL_CLOSE without having sent or received
   SSH_MSG_CHANNEL_EOF." suggests to me the check for state EOF is problematic. Certainly I was getting connection errors because the backed sshd had used up all the channels with CLOSE messages missing in action due to this original clause.