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.

tonic-health/Cargo.toml

@@ -0,0 +1,30 @@
+[package]
+name = "tonic-health"
+version = "0.1.0"
+authors = ["James Nugent <[email protected]>"]
+edition = "2018"
+license = "MIT"
+repository = "https://github.com/hyperium/tonic"
+homepage = "https://github.com/hyperium/tonic"
+description = """
+Health Checking module of `tonic` gRPC implementation.
+"""
+readme = "README.md"
+categories = ["network-programming", "asynchronous"]
+keywords = ["rpc", "grpc", "async", "healthcheck"]
+
+[features]
+transport = ["tonic/transport"]
+
+[dependencies]
+async-stream = "0.2"
+tokio = { version = "0.2", features = ["sync", "stream"] }
+tonic = { path = "../tonic", features = ["codegen", "data-prost"] }
+bytes = "0.5"
+prost = "0.6"
+
+[dev-dependencies]
+tokio = { version = "0.2", features = ["rt-core", "macros"]}
+
+[build-dependencies]
+tonic-build = { path = "../tonic-build" }