This doesn't add any new extensibility, and will likely change again in
the future when we add extensibility. For now this is a stopgap to reduce
duplication between JsonResult and JsonViewComponentResult.
- move `DynamicObject` derivation up to new `DynamicViewData` class, fixing [WebFx-169](http://projectk-tc:8080/browse/WEBFX-169)
- avoid direct `_data` lookup in previous `TryGetMember()`, fixing [#118](https://github.com/aspnet/WebFx/issues/118)
- rename ViewData -> ViewDataDictionary
Also
- flesh out `IDictionary<string, object>` implementation in `ViewData`
- provide `ViewData` copy constructor that allows TModel to change
- remove `TryGetIndex()` and `TrySetIndex()` implementations; use `ViewData[]` instead
- restore `ViewContext.ViewBag` from legacy MVC
N. Taylor Mullen
Ryan Nowak
dougbu