Use generics to provide an transformer based interface for custom types

[kylef]

The idea behind this is that we can use Turnstone with custom transformers making it easy to mix and match Turnstone with Nest-based middleware and web frameworks which might use custom types.

Theoretical example:

struct Request {
  let uri:String
  let parameters:[String:String]
}

var turnstone = Turnstone({request in request.uri}, undefined, undefined)

turnstone.addRoute('/questions/{id}') { request in
  let id = request.parameters["id"]
  return Response("Question \(id)")
}

turnstone.route  // takes a "Request" structure and expects a "Response" structure to be returned