Issue #12 - Support ssh gateways.

Looking for feedback...

This commit provides support for connecting to the destination host via an SSH gateway.
- connector_port_open? implementation for winrm and ssh
- the ssh implementation checks if it can connect to the gateway if the gateway is given. If
  it is not given then connect directly to the destination.
- host_commander#connector_for now calls the connector_port_open? for winrm and then ssh.
- port_open? moved to host_connector so it could be called from the winrm and ssh host connectors.
- ssh connector's run method now creates a gateway connection if the gateway value is in the :ssh options.