Apollo subgraph example

[pavelnikolov]

fixes: #602
fixes: #528

In order to test:

1. Pull the master branch
2. Run cd example/apollo_federation/subgraph
3. Run go run server.go
4. Open http://localhost:8080 in your browser
5. Test with a query like this:

    query subgraph{
      post(id: "3") {
        id
        title
      }
      _entities(representations: ["{\"__typename\": \"Post\", \"id\": \"3\"}"]) {
        __typename
        ... on Post {
          id
          title
        }
      }
      _service {
        sdl
      }
    }