Fix for #617 - ViewResult can be constructed without a view engine

We want result types to be constructable without services. This will simplify testing for end users and POCO controllers.
According to https://github.com/aspnet/Mvc/issues/719, we need to change ViewResult to allow multiple IViewEngines to be passed in (probably via MvcOptions). In which case, do we envision allowing people to specify a sequence of engines they want to use for discovery?