add http health checks.

@smarterclayton was making the point about implementing your own health checks, I could imagine having a centralized health monitoring service container that monitors and provides health for all of the other pods in your application.  So in that case

Host: <constant-host-name>
Path: /some/path/to/checker?container=foo

so you could provide centralized logic for determining health.  I know seems a little far-fetched, but I think the extensibility is useful.