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
> Can you explain where the "request" in variable names like $request_forwarding comes from? Is this process defined as request forwarding in some technical documentation?

The way it looks like it works is... you request agent forwarding and then a new channel is created. If an interactive shell then expects a signature with your private key it sends the data it needs you to sign to you via this new channel and then you reply with the signed data. The SSH server then sends this off.

One thing I do wonder tho is how well this works with multiple interactive shell channels. Right now, however, it's a non-issue as you can only have one interactive shell channel with phpseclib, currently.