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.mdFixes#135.
