Fix StyleCop issues
[Issue #527] Revive common ActionResults - Part 1.

1. Added HttpNotFound() to Controller.
2. Updated HttpStatusCodeResult to expose the StatusCode as a property.
3. Added unit tests for HttpNotFound() and for HttpStatusCodeResult.
4. Updated the MvcSample to add an action that uses HttpNotFound().
5. Brought back HttpNotFoundResult and added unit tests for it.
Fix #620, Remove legacy / non-unobtrusive client side validation
- remove `ViewContext.UnobtrusiveJavaScriptEnabled` property and all references
- avoid `ViewContext.GetFormContextForClientValidation()` calls since
  ternary expression is more explicit and we were inconsistent
- improve `ValidationMessage()` comments
- don't treat `ModelState.IsValid` as if it were still nullable
Cleanup unused `FormContext` members due to legacy validation removal
- provide new property back for customer use
- remove unused FieldValidationMetadataClass
1. Introducing XML Input Formatters.
2. Adding DelegatingStream class
3. Unit + Functional tests for formatters.
Changes to enable XML Output Formatters.