add healthcheck

[ThomasDonath]

great utility!

If I'll use it in a cloud with a public endpoint I must use it with authentication. This works well, but a cloud load balancer does need a healthcheck endpoint without authentication. So I've patched server.go

80a81,86
> 		// TDO
> 		if req.URL.Path == "/healtz" || req.URL.Path == "/healtz/" {
> 			w.WriteHeader(200)
> 			return
> 		}
>       // end TDO

As I can't open a pull request I'll leave here. Would be great if accepted. Thanks!