Add resolver middleware Add opentracing support

[vektah]

Adds a user specified resolver middleware stack and some opentracing resolver middleware that uses it.

The middlware stack probably goes a long way towards #39, allowing middlware to be defined around the user defined resolvers. It will not be called around any directly bound models fields or methods.

example usage:

	http.Handle("/query", handler.GraphQL(starwars.MakeExecutableSchema(starwars.NewResolver()),
		handler.Use(func(ctx context.Context, next graphql.Resolver) (res interface{}, err error) {
			rc := graphql.GetResolverContext(ctx)
			fmt.Println("Entered", rc.Object, rc.Field.Name)
			res, err = next(ctx)
			fmt.Println("Left", rc.Object, rc.Field.Name, "=>", res, err)
			return res, err
		}),
	))

weakly typed interface but there is an assertion in the generated code as soon at it returns. This is mostly to allow access to the error on the way through. Unmarshaled args are now in resolver context, and can be manipulated before calling next.

The dataloader example now hosts an appdash instance to see the metrics collected. Here is an example of running

query {
  customers {
    name
    address {
      street
    }
    orders {
      date
      items {
        name
      }
    }
  }
}