feat(health): Add tonic-health server impl

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.

examples/README.md

@@ -72,6 +72,13 @@ $ cargo run --bin tls-client
 $ cargo run --bin tls-server
 ```
 
+## Health Checking
+
+### Server
+
+```bash
+$ cargo run --bin health-server
+```
 
 ### Notes: