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/Cargo.toml

@@ -102,6 +102,10 @@ path = "src/hyper_warp/client.rs"
 name = "hyper-warp-server"
 path = "src/hyper_warp/server.rs"
 
+[[bin]]
+name = "health-server"
+path = "src/health/server.rs"
+
 [dependencies]
 tonic = { path = "../tonic", features = ["tls", "data-prost"] }
 prost = "0.6"
@@ -126,6 +130,8 @@ warp = { version = "0.2", default-features = false }
 http = "0.2"
 http-body = "0.3"
 pin-project = "0.4"
+# Health example
+tonic-health = { path = "../tonic-health" }
 
 [build-dependencies]
 tonic-build = { path = "../tonic-build", features = ["prost"] }