Add more examples

[Jonathan Rezende]

*This issue was automatically moved to: angel-dart/angel#208.

[Tobe Osakwe]

The graphql_server package implementation is probably the best example; it interacts with every part of the GraphQL tree:
https://github.com/angel-dart/graphql/blob/master/graphql_server/lib/graphql_server.dart

I'll keep this open, though, because more examples couldn't hurt.