Added mbus listener and responds to director pings

Signed-off-by: Jeffrey Peckham <peckham@pivotallabs.com>
Added handling of SIGTERM and SIGINT to stop the agent

Signed-off-by: Damien Le Berrigaud <damien@pivotallabs.com>
Move ssh setup into infrastructure and platform

 * WrapError in bosh/errors

Signed-off-by: Damien Le Berrigaud <damien@pivotallabs.com>
Introduce dummy Infrastructure and Platform for local testing

Signed-off-by: Damien Le Berrigaud <damien@pivotallabs.com>