fix(ssh): Use DialTCP instead of Dial

It helps to set raddr(remote address) in net.Conn, which is later used
in HostKeyCallBack.
Merge pull request #233 from unihorn/48

fix(ssh): Use DialTCP instead of Dial
feat(sshAgentForwarding): Allow to forward authentication requests to an agent.
Merge pull request #228 from calavera/ssh_agent_forwarding_feature

feat(sshAgentForwarding): Allow to forward authentication requests to an agent.
fix(ssh.Execute): Handle ssh execution errors.