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.

Cargo.toml

@@ -2,9 +2,12 @@
 members = [
     "tonic",
     "tonic-build",
+    "tonic-health",
+
     # Non-published crates
     "examples",
     "interop",
+
     # Tests
     "tests/included_service",
     "tests/same_name",