Add ITagHelperActivator.

- The TagHelperActivator enables dependency injection via properties and allows access to the ViewContext.
- This replaces the ICanHasViewContext mechanism that we had in place before.
- Added tests and fixed up existing to work with new format for providing ViewContext.

#1258
Add Microsoft.AspNet.Mvc.Test.Common project.

- Added a common test to share common test files.  Moved the TestFileSystem from the razor host project so we could utilize it in other test projects.

#1268