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
Add ModelExpression code generation.

- Sealed the ModelExpression.
- We use the stringified version of the ModelExpression type name to detect ModelExpression properties on TagHelpers. This is so the MvcRazorHost can work in tooling and in runtime.
- Created a GeneratedTagHelperAttributeContext to represent the specific stringified versions of the ModelExpression assets.
- Created an MvcTagHelperAttributeValueCodeRenderer to modify rendering of ModelExpression properties.

#1241
Add tests to validate attribute value rendering.

- Exposed internals from Mvc.Razor.Host to Mvc.Razor.Test so it can use the MvcRazorHost override that takes the IFileSystem.
- Added end-to-end code generation tests for TagHelpers with ModelExpression properties.

#1241