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
     }
   }