Send RPC from skinny server

[iandioch]

Closes #55.

Get gRPC working on server. Shown here by contacting the greeting card example service.

[devoxel]

Looks grand, one comment about how you're creating the connection

[devoxel]

In general, we shouldn't be creating grpc connections everytime we open a response, instead we should use an existing connection (via a client), and only make the connection once when the server is started.

we may want an s.greeter that is the client, so it's easy to pass a mocked client in during a test

[iandioch]

s/response/request?

You're probably right, and I like the idea of s.greeter. That fits the problem I was trying to figure out with passing in a mock in tests. I will change this.

[iandioch]

I'd like to reformat the RPC services in the serverWrapper into some common struct or interface, so that there's just something like:

type RPCService struct {
  conn *grpc.ClientConn
  service pb.blahBlahClient
}

in the serverWrapper for each RPC service. This seems cleaner than holding a conn and a client for each individual RPC endpoint individually in the serverWrapper. However, because stuff with generics doesn't really work in Go, it's tough to do this with in a nice way for every blahBlahClient without losing type safety.

I'll hopefully play with this again in future to find a nice way, but in the meantime, PTAL a this PR.