SSH2: add ssh-agent support
SSH_Agent: misc updates
SSH_Agent: phpdoc updates
Merge pull request #265 from terrafrost/ssh-agent

SSH2: add ssh-agent support

* terrafrost/ssh-agent:
  SSH_Agent: phpdoc updates
  SSH_Agent: misc updates
  SSH2: add ssh-agent support
Add newline at EOF to phpseclib/System/SSH_Agent.php.