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
So what happens if phpseclib (ie. not the server) killed the connection? phpseclib [sends `NET_SSH2_MSG_CHANNEL_CLOSE`](https://github.com/phpseclib/phpseclib/blob/96eb1f379797bca03452f2bec33315dd4d289bd4/phpseclib/Net/SSH2.php#L3078), `$this->channel_status[$channel]` is set to `NET_SSH2_MSG_CHANNEL_EOF` and then the server response with `NET_SSH2_CHANNEL_CLOSE` and phpseclib responds with `NET_SSH2_CHANNEL_CLOSE`, when it already sent one? So phpseclib should be trying to close a channel that's already closed? That just seems like a bad idea. Also, FYI the code I linked to is the only place where `$this->channel_status[$channel]` is set to `NET_SSH2_MSG_CHANNEL_EOF`.

If you're getting connection errors maybe post the logs. Or maybe give me SSH access and I'll perform the diagnostics.