User entities and contexts

User Entities and Contexts can be added to the request with such code:

var myDwarfs = new Entity("dwarfs");
myDwarfs.AddEntry(new EntityEntry("Ori", new [] {"ori", "Nori"}));
myDwarfs.AddEntry(new EntityEntry("bifur", new [] {"Bofur","Bombur"}));
var extraEntities = new List<Entity> { myDwarfs };

var extraContexts = new List<AIContext> { new AIContext { Name = "someContext" } };

apiAi.TextRequest("hello", requestExtras);