Tag helpers parse tree rewriter tests

- Added tests to validate TagHelperParseTreeVisitor when dealing with valid HTML elements.
- Added understanding to the ParserTestBase for TagHelperBlock's
- Added a helper class MarkupTagHelperBlock to make building test TagHelpers easier.
- Fixed some existing tests that "new"d up the RazorParser and didn't obide by the new contract (to provide optimizers).

The implementation portion of this is in a separate PR here: https://github.com/aspnet/Razor/pull/109


Issue: #71
No, this simplifies the TagHelperBlock even more than is exposed.