This commit adds a new crate `tonic-health` which implements the [standard GRPC Health Checking][checking] protocol. Currently there is only a server implementation, though others have alluded in the discussion in #135 that client implementations exist which could also be imported as necessary. A example server has also been added - once the client work is done a client for this should be added also. [checking]: https://github.com/grpc/grpc/blob/master/doc/health-checking.md Fixes #135.